THE  LIBRARY 

OF 

THE  UNIVERSITY 

OF  CALIFORNIA 

LOS  ANGELES 


TRANSLATED  FROM  THE  DANISH 

BY 

ELISABETH  AAGESEN 


CENTRilTRYKKERICT- NYKOBINO  F. 


1 


MicUl  Loutl-fliU 


yix4/2AA^  /h.A 


SKIASCOPY 


AND  ITS 


Practical  Application  to  the 
Study  of  Refraction 


BY 


Edward  Jackson,  a.  m.,  m.  d., 

PROFESSOR    OF    DISEASES    OF    THE     EYE    IN    THE    PHILADELPHIA    POLYCLINIC  AND   COLLEGE    FOR 

GRADUATES  IN  JIEDICINE  ;    SURGEON  TO  WILLS  EYE  HOSPITAL;    CHAIRMAN  OF  THE 

SECTION  ON  OPHTHALMOLOGY  OF  THE  AMERICAN  MEDICAL  ASSOCIATION  ; 

MEMBER  OF  THE  AMERICAN  OPIITHALMOLOGICAL  SOCIETY  ; 

ETC.,  ETC. 


WITH  TWENTY-SIX  ILLUSTRATIONS 


PHILADELPHIA: 

THE  EDWARDS  &  DOCKER  CO. 

1895 


COPYRIGHT,  i8o5 
BY  EDWARD  JACKSON 


CONTENTS 


PREFACE    

CHAPTER  I.— History,  Name,  Difi--icui,ties  and  Study 

History        .         . 

Name  .         .  .         .    '     . 

Difficulties 

How  to  Stud}'  the  Test       ...... 

CHAPTER  II.— General  Opticai.  Principles. 
The  reversal  of  movement 

Real  Movement  of  light  on  retina,  Plane  Mirror 
Real  Movement  of  light  on  retina.  Concave  Mirror 
Apparent  ISIovement  of  light  in  pupil 
Rapidity  of  Movement  of  light  on  the  retina 
INIagnification  of  the  retina 
Form  of  the  light  area        .... 
Brilliancy  of  light  in  the  pupil 
Finding  the  point  of  reversal    . 

CHAPTER  III— Conditions  oe  Accuracy. 

The  Source  of  light   ..... 
Focusing  of  light  on  the  retina 
I'osition  of  greatest  accuracy     . 
Irregularities  of  the  INIedia  and  Surfaces 
Distance  of  Surgeon  from  Patient     . 

CHAPTER  IV.— Regular  .\stigmatism. 

Two  points  of  reversal       ....... 

The  Band-like  appearance 

Changes  in  the  Light  area  at  different  distances 
Direction  of  movement  of  the  band 

CHAPTER  V. — Aberration  and  Irregular  Astigmati.sm 
Appearance  of  Irregular  .Astigmatism       .... 

Synmietrical  Aberration     ...... 

The  Visual  Zone         ........ 

Appearance  of  Positive  .Vljerration   ..... 


7 
lO 

II 
13 

19 
23 
24 
26 
28 
30 
32 
33 
34 

36 
3S 
40 
41 
43 

46 
47 
53 
55 

56 

5S 
59 
60 


Appearances  of  Negative  Aberration  .....  63 

Appearance  of  Conical  Cornea 65 

Scissors-like  Movement     . 68 

CHAPTER  VI.— Practical  Application  with  Plane  Mirror. 

Position  and  Arrangement  of  Light  .         .         ,         .         .71 

Hyperopia  ...........  72 

Myopia 74 

Emmetropia 76 

Regular  Astigmatism 77 

Aberration  and  Irregular  Astigmatism 84 

Measurement  of  Accommodation 86 

CHAPTER  VII.  —  Practical  Application  with  Concave  Mirror. 

The  Source  of  Light 89 

Hyperopia  ..........  90 

Myopia 91 

Emmetropia        ...  ......  93 

Regular  Astigmatism 93 

Aberration  and  Irregular  Astigmatism 99 

Measurement  of  Accommodation 99 

CHAPTER  VIII  —General  Considerations. 

Apparatus 100 

Mydriatics 106 

Relative  advantages  of  plane  and  concave  mirrors  .  .  107 

INDEX Ill 


PREFACE 


THIS  little  book  was  written  to  bring  about  the  more 
general  adoption  of  Skiascopy  as  an  essential  part  of 
the  examination  for  ametropia.  It  is  not  supposed  that 
any  ophthalmologist  is  quite  ignorant  of  the  test ;  but 
many  do  not  know  its  full  practical  value,  or  how  best  to 
apply  it. 

The  demonstrations  and  descriptions  here  given  assume 
a  general  knowledge  of  the  eye  and  of  physiological  optics. 
And  the  writer,  having  observed  that  students  of  this 
subject  do  not  generally  think  in  the  terms  of  algebraic 
formulas,  but  more  readily  grasp  the  graphic  or  geometric 
presentation  of  a  fact,  has  governed  himself  accordingly. 

The  claims  of  this  subject  to  careful  consideration  are  : 

First. — Skiascopy,  is  an  objective  test,  independent 
of  the  patient's  intelligence  or  visual  acuteness,  and  more 
largely  than  any  other,  independent  of  the  patient's  coopera- 
tion. 

Second. — It  is  by  far  the  most  accurate  objective 
test.  The  limits  of  its  accuracy  depend  on  details  of  its 
execution,  and  the  skill  and  patience  of  the  observer ;  but, 
it  does  not  require  any  rare  natural  qualifications,  to  carry 
it,  for  many  eyes,  to  the  extreme  limits  of  accuracy  for 
subjective  tests. 

Third. — It  requires  but  little  more  time  than  the  use 
of  the  refraction  ophthalmoscope  or  the  ophthalmometer, 
which  are  able  to  give  very  inferior  information.  It  saves 
time  in  making  a  complete  diagnosis. 


Fourth. — It  requires  no  costly,  complex  or  cumber- 
some apparatus. 

Fifth. — It  lays  before  the  surgeon  the  refraction  in 
each  particular  part  of  the  pupil  as  it  is  revealed  by  no 
other  test,  and  opens  up  the  principal  avenue  for  farther 
advance  in  the  scientific  study  of  the  refraction  of  the  eye. 

Of  the  use  of  the  data  obtained  by  means  of  skias- 
copy, it  is  not  the  purpose  of  the  present  monograph  to 
speak.  These  data  include  the  refraction  of  the  visual 
zone,  corresponding  to  the  refraction  of  the  eye  obtained 
by  other  methods ;  an  accurate  knowledge  as  to  the  loca- 
tion and  limits  of  that  zone ;  and  the  refraction  outside  of 
it ;  the  latter  having  in  some  cases  important  bearings  on 
the  practical  adjustment  and  use  of  lenses. 

Denver,   Col.,  March,   1895. 


CHAPTER  I. 

HISTORY,    NAME,    DIFFICULTIES,     AND    METHOD   OF   STUDY- 
ING  THE   TEST. 

History. — From  the  earliest  use  of  the  ophthalmoscope, 
by  the  direct  method,  it  has  been  recognized  that  the  see- 
ing of  an  erect  image  of  the  fundus  at  some  little  distance 
from  the  eye  indicated  hyperopia,  and  the  seeing  of  an  in- 
verted image  indicated  myopia.  So  long  ago  as  1862, 
Bowman  {The  Royal  London  Opthalmic  Hospital  Reports, 
Vol.  II,  p.  157),  called  attention  to  the  rotation  of  the  mir- 
ror as  a  means  of  bringing  out  appearances  characteristic 
of  irregular  astigmatism  and  conical  cornea,  and  Bonders, 
in  his  work  on  Accommodation  and  Refraction  of  the  Eye,  pub- 
lished in  1864,  included  (p.  490)  the  following  note: 

*'  My  friend  Bowman  recently  informs  me  that  '  he 
has  been  sometimes  led  to  the  discovery  of  regular  astig- 
matism of  the  cornea,  and  the  direction  of  the  chief  meridi- 
ans, by  using  the  mirror  of  the  ophthalmoscope  much  in 
the  same  way  as  for  slight  degrees  of  conical  cornea.  The 
observation  is  more  easy  if  the  optic  disc  is  in  the  line  ot 
sight  and  the  pupil  large.  The  mirror  is  to  be  held  at  two 
feet  distance,  and  its  inclination  rapidly  varied,  so  as  to 
throw  the  light  on  the  eye  at  small  angles  to  the  perpen- 
dicular, and  from  opposite  sides  in  succession,  in  successive 
meridians.  The  area  of  the  pupil  then  exhibits  a  some- 
what linear  shadow  in  some  meridians  rather  than  in 
others.'  " 

(7) 


8  SKIASCOPY. 

The  use  of  the  ophthalmoscope  above  referred  to,  for 
the  detection  of  irregular  astigmatism,  became  widely  pop- 
ular. It  was  generally  adopted  as  the  most  satisfactory  test 
for  this  kind  of  defect.  But  the  observation  that  the  same 
method  was  capable  of  revealing  regular  astigmatism  and 
the  direction  of  its  principal  meridians,  does  not  seem  to 
have  attracted  the  same  attention. 

In  1872,  Couper,  in  his  paper  before  the  Fourth  Inter- 
national Ophthalmol ogical  Congress  (see  Trans,  page  109), 
alluded  to  Bowman's  observations,  and  said  :  "  The  greater 
dispersion  in  one  meridian  than  in  the  opposite,  gives  rise 
to  the  linear  shadows.  Only  the  fact  of  astigmatism  is 
thus  established."  He  then  went  on  to  describe  a  method 
of  using  the  ophthalmoscope  as  an  optometer  in  astigma- 
tism, which  is  rather  a  modification  of  the  ordinary  use  ot 
the  ophthalmoscope  than  a  variety  of  skiascopy,  since  it 
depends  on  the  recognition  or  non-recognition  of  the  retinal 
vessels  in  different  meridians,  when  the  ophthalmoscope 
mirror  is  held  at  a  considerable  distance  from  the  eye,  and 
takes  no  account  of  the  movement  of  a  light  area  on  the 
retina.  It  had  already  been  pointed  out  (Bonder's  Accom. 
and  Eef.  ojthe  Eye,  page  106),  that  the  distance  of  the  in- 
verted image  before  the  eye  indicated  the  degree  of  myopia. 

In  1873,  Cuignet,  of  Lille,  published  (Rec.  d'Ophtal- 
mol,  1873,  pp.  14  and  316)  an  account  of  the  test,  as  he 
had  used  it,  as  one  capable  of  revealing  not  only  the  pres- 
ence of  hyperopia  or  myopia  as  well  as  astigmatism,  but 
also  as  giving  a  practical  method  of  measuring  the  amount 
of  these  errors  of  refraction.  He  seems  not  to  have  appre- 
ciated fully  the  optical  principles  involved  in  the  test,  and 
his  account  of  it  attracted  no  attention.  However,  in  1878, 
his  pupil,  Mengin,  introduced  the  practice  of  the  method 
at  Galezowski's  clinic  in  Paris.     There  it  was  taken  up, 


HISTORY.  9 

and  Parent  demonstrated  its  true  optical  basis  and  urged 
its  advantages  in  a  series  of  articles  published  in  the  Recucil 
d' Ophtahnologie  in  1 880-81,  pp.  65  and  229. 

Lytton  Forbes,  in  the  Royal  London  Ophthalmic  Hos- 
pital Reports  for  1880,  (p.  62),  published  a  paper  on  the  test, 
giving  a  minute  account  of  the  various  forms  assumed  by 
the  light  and  shadow  in  the  pupil,  but  without  full  expla- 
nation of  their  optical  significance.  In  1881,  A.  Stanford 
Morton  included  a  full  description  of  the  test  in  his  little 
work  on  the  Refraction  of  the  Eye.  In  1882,  Charnley  gave 
the  fullest  demonstration  of  its  optical  basis  in  the  Royal 
London  OphtJialmic  Hospital  Reports^  X,  3,  p.  344.  And  Juler 
called  attention  to  it  in  the  Ophthalmic  Review  Vol.  I,  p.  327. 

The  method  described  and  advocated  by  Parent  and 
those  who  followed  him,  had  been  that  with  the  concave 
mirror.     Cuignet  had  used  the  plane  mirror,  and,  in  1882, 
Chibret  pointed  out  {Annales  d^  Oculistiqiie^  Vol.  xxxviii,  p. 
238)  the  advantages  of  the  plane  mirror  in  determining  the 
presence  and  degree  of  myopia  in  the  examination  of  large 
numbers  of  recruits.     In  1883,  Story  {Ophthalmic  Review^ 
Vol.  II,  page  228)  advocated  the  use  of  the  plane  mirror,, 
but  in  the  same  manner  as  the  concave,  except  that  the  ob- 
server should  place  himself  at  a  distance  of  four  metres, 
from  the  patient,  a  distance  which  renders  the  test  of  little 
value  for  a  considerable  proportion  of  cases. 

In  1885,  was  published  {American  Journal  of  the  Medical 
Sciences^  April,  1885)  the  writer's  account  of  the  test  with 
the  plane  mirror,  as  applicable  to  all  varieties  of  ametropia, 
the  determination  being  made  by  measuring  the  variable 
distance  of  the  surgeon  from  the  patient.  Since  that  time 
the  test  has  been  widely  recognized,  but  even  yet  is  far 
from  being  universally  adopted  and  depended  upon  as  it 
deserves  to  be.  The  literature  of  the  subject  has  since  grown 
1 


10  SKIASCOPY. 

quite  extensive.  But  it  must  be  noted  that  a  considerable 
proportion  of  the  accounts  of  the  test  bear  evidence  that 
their  author's  acquaintance  with  it  has  been  theoretical 
rather  than  practical,  and  the  mass  of  them  contribute 
nothing  to  the  common  fund  of  professional  knowledge. 
The  writer's  contributions  as  to  the  retinal  illumination 
(Ophthalmic  Review^  Feb.,  1890)  and  the  relative  positions 
of  the  source  of  light  and  the  observer  {Archives  of  Ophthal- 
mology, July,  1893),  with  the  suggestions  as  to  the  special 
pieces  of  apparatus  to  facilitate  the  test,  to  be  mentioned 
in  Chapter  VIII,  complete  the  evolution  of  this  method  of 
diagnosis  as  now  practiced,  and  here  described. 

Name  of  the  test. — Neither  Bowman  nor  the  others, 
who  early  employed  the  test  for  the  detection  of  irregular 
astigmatism  and  conical  cornea,  proposed  for  it  any  special 
name. 

Cuignet,  who  brought  it  forward  as  a  distinct  method  for 
the  diagnosis  of  the  refraction  of  the  eye,  seems  to  have 
thought  at  first  that  the  play  of  light  and  shade  in  the 
pupil  depended  entirely  on  the  curvature  of  the  cornea, 
and  described  it  under  the  name  keratoscopie.  Considering 
the  real  causes  of  the  movement  of  light  and  shade  in  the 
pupil  and  the  purposes  for  which  it  is  employed,  this  name 
seems  especially  inappropriate. 

Parent,  realizing  this  inappropriateness,  proposed  retino- 
scopie,  in  allusion  to  the  fact  that  it  was  the  movement  of 
light  and  shade  on  the  pigment  layer  of  the  retina  that 
commonly  gave  rise  to  the  phenomena  studied.  Yet  this 
name  was  obviously  open  to  criticism,  in  that  the  condition 
of  the  retina  itself  was  not  at  all  the  matter  in  considera- 
tion, and  that  the  same  play  of  light  and  shade  could  be 
watched  on  the  head  of  the  optic  nerve,  or,  where  the  reti- 
nal pigment  was  wanting,  upon  the  choroid  or  sclera. 


HISTORY.  11 

Cliibret,  to  bring  out  the  point  that  it  was  the  movement 
of  a  shadow  that  was  the  subject  of  investigation,  proposed 
the  name  of  fantoscopie  retinienne,  and,  Mr.  Priestley  Smith, 
probably  anglicizing  this  term  and  dropping  the  allusion 
to  the  retina,  called  it  the  shadow-test.  This  name  though 
a  compound  word,  where  a  simple  one  should  do,  became 
extremely  popular,  and  its  appropriateness  led  Chibret  to 
call  to  his  aid,  the  linguistic  skill  of  M.  Egger,  who  ren- 
dered it  in  the  term  skiascopia,  which  in  its  French  form 
skiascopie,  or  its  English  form,  skiascopy,  has  been  most 
widely  accepted  as  the  proper  term  to  designate  the  test. 

Umbrascopy,  proposed  by  Hartridge,  is  indefensible  on 
linguistic  grounds,  and  the  same  is  true  of  papilloscopie 
proposed  by  Landolt,  and  for  which  he  afterwards  offered 
the  equivalent,  koroscopie.  Dioptroscopie  was  advocated  by 
Galezowski  {Atlas  d' Ophthalmoscopie)  and  is  appropriate, 
though  equally  applicable  to  other  methods  of  measuring 
refraction. 

Retinophotoscopie  and  retinoskiascopie  have  have  also  been 
recently  suggested  by  Parent,  but  there  seems  to  be  no  suf- 
ficient reason  for  retaining  in  the  name  any  allusion  to  the 
retina.  Fundus-reflex-test  suggested  by  Oliver  is  also  unnec- 
essarily long  for  a  name. 

The  suggestion  has  sometimes  been  made  to  apply  one 
of  these  names  to  one  form,  and  another  name  to  another 
form  of  the  test.  But  such  a  use  of  them  is  not  warranted 
by  their  original  suggestion  or  by  custom,  nor  is  there  any 
sufficient  reason  for  the  employment  of  separate  names  to 
differentiate  various  forms  of  the  test.  In  all  its  different 
forms,  the  test  is  essentially  the  same  ;  the  difference  being 
merely  as  to  the  apparatus  and  mechanical  detail. 

Difficulties  of  the  Test — That  skiascopy,  though  a  val- 
uable method  of  examination,  is  one  difficult  to  master, 


12  SKIASCOPY. 

becomes  more  and  more  evident  as  one  continues  to  work 
with  it.  The  theoretical  basis  is  perfectly  simple  ;  the 
fundamental  phenomena  readily  observed  ;  and,  with  a  few 
days  practice,  the  merest  tyro  may  be  able  by  it  to  estimate 
the  refraction  in  favorable  eyes  with  an  accuracy  not  to  be 
attained  by  any  other  objective  method.  But  long  after 
the  stage  of  such  acquirement  has  been  passed,  the  surgeon 
will  again  and  again  encounter  cases  that  still  prove  diffi- 
cult and  puzzling.  Nothing  but  a  thorough  understanding 
of  the  optical  principles  involved,  and  patient  study  of  the 
eyes  which  prove  most  puzzling,  under  carefully  arranged 
favorable  conditions,  will  enable  one  to  master  the  test. 

The  importance  of  careful  arrangement  of  the  relative 
positions  of  the  light  and  of  the  observer,  and  the  adapta- 
tions of  the  mirror  have  not  heretofore  been  sufficiently 
insisted  upon.  What  these  adaptations  and  arrangements 
are  will  appear  under  their  proper  headings  in  chapters  III 
and  IV.  It  is  here  only  necessary  to  emphasize  their 
importance.  For  instance  :  All  descriptions  of  the  shadow- 
test  allude  to  the  characteristic  band-like  appearance  of  the 
light  in  astigmatism.  Now,  as  a  matter  of  fact,  even  in 
the  highest  degrees  of  astigmatism,  such  an  appearance 
cannot  be  perceived,  except  with  certain  lenses,  or  at  cer- 
tain distances  in  front  of  the  eye ;  and  it  is  a  distinctive 
and  exact  indication  only  when  the  light,  the  mirror,  and 
the  patient's  and  the  observer's  eyes  are  brought  into  a 
certain  relation. 

It  would  be  as  rational  to  attempt  to  measure  refraction 
with  an  ophthalmoscope  devoid  of  any  lens  series,  or  to  test 
the  acuteness  of  vision  in  a  darkened  room,  as  to  expect 
definite  and  satisfactory  results  from  skiascopy,  applied 
without  careful  attention  to  details  that  have  usually  not 
been  referred  to  in  descriptions  of  the  test. 


DIFFICULTIES.  13 

The  fact  that  this  test  shows,  as  does  no  other,  the  actual 
refraction  of  the  eye  for  each  particular  portion  of  the 
pupil,  increases  enormously  the  wealth  of  phenomena  it 
offers  for  study,  adding  to  its  scientific  and  practical  value, 
but  also  making  it  more  difficult  by  rendering  it  necessary 
to  discriminate  between  the  particular  portions  of  the 
movement  of  light  and  shade  which  are  of  practical  im- 
portance, and  others  which  are  not. 

How  to  Study  the  Test. — The  study  of  skiascopy  is 
something  quite  different  from  its  practical  application.  To 
start  from  a  few  bare  rules  as  to  the  placing  of  glasses,  and 
the  movements  of  the  mirror,  and  the  light  in  the  pupil, 
and  attempt  to  learn  the  test  by  using  it  will  never  give  a 
mastery  of  it.  It  is  better  to  make  a  careful  study  of  it 
before  attempting  to  employ  it  as  a  method  of  ascertaining 
the  refraction. 

Such  a  study  is  chiefly  a  use  of  the  test,  but  from  a 
standpoint  entirely  different  from  that  of  its  application  in 
practice.  To  study  the  test,  one  should  as  far  as  possible, 
start  with  known  conditions  of  refraction,  with  lenses  of 
known  strength,  with  the  eye  at  a  known  distance,  and 
should  observe  the  character  of  the  movements  of  light  and 
shadow  in  the  pupil,  which  belong  to  these  known  condi- 
tions. He  should  work  from  known  refraction  to  the 
pupillary  appearances  that  belong  to  it.  While  in  using 
the  test  for  the  measurement  of  ametropia,  he  has  to  deduce 
from  observed  pupillary  appearances  the  state  of  refraction 
causing  them. 

The  student  may,  from  time  to  time,  test  his  progress 
towards  proficiency  by  attempts  to  measure  refraction  by 
skiascopy,  but  familiarity  with  the  appearances  indicative 
of  known  conditions  of  refraction  is  chiefly  to  be  sought. 

The  appearances  upon  which  the  attention  is  fixed  in 


.14  SKIASCOPY. 

skiascopy  are  those  of  the  red  reflex  in  the  pupil.  The 
first  step  is  to  learn  just  what  this  appearance  is  and  some 
of  the  variations  of  which  it  is  capable.  Let  the  beginner 
with  his  eye  at  the  sight-hole  of  the  skiascopic  mirror 
throw  into  the  observed  eye,  from  a  distance  of  20  or  30 
inches,  the  light  from  a  lamp  flame  as  in  the  ordinary  oph- 
thalmoscopic examination.  Looking  into  the  observed  eye 
with  the  light  properly  directed,  he  will  see  the  brilliant 
point  of  light,  the  reflection  from  the  surface  of  the  cornea 
of  the  lamp  flame  he  is  using  ;  and  he  may  also  see  reflections 
of  his  own  face  or  of  other  objects  from  the  surface  of  the 
cornea.  These  are  to  be  disregarded.  The  real  object  of 
study,  the  phenomena  upon  which  attention  is  to  be  fixed, 
is  the  general  red  glow  perceived  within  the  pupil,  the 
fundus  reflex. 

If  the  mirror  be  rotated  about  an  axis  lying  in  the  plane 
of  the  mirror,  the  area  of  light  thrown  by  it  upon  the 
face  will  move  in  the  direction  towards  which  the  mir- 
ror is  turned.  As  the  test  becomes  familiar,  the  direction 
of  this  movement  will  be  known  without  any  conscious 
effort  to  discover  it.  With  the  concave  mirror  at  a  greater 
or  lesser  distance  than  its  focus  or  with  the  plane  mirror  at 
all  distances,  except  at  the  point  of  reversal  which  it  is  the 
object  of  the  test  to  determine,  the  rotation  of  the  mirror 
also  causes  a  movement  of  the  red  reflex  in  the  pupil.  As 
the  reflex  disappears  from  the  pupil,  it  is  followed  by  an 
area  of  shadow,  and,  as  it  returns  to  the  pupil,  the  shadow 
passes  out  before  it.  The  movement  of  the  light  area 
really  goes  on  when  no  shadow  is  visible  in  the  pupil,  but 
only  when  light  and  shade  are  both  seen  can  the  movement 
be  recognized.  We  know  the  movement  of  light  in  the 
pupil  by  the  movement  of  the  boundary  between  light  and 
shade. 


STUDY  OF  THE  TEST.  15 

Having  learned  what  it  is  that  he  has  to  watch  in  the 
pupil,  the  student  should  make  himself  familiar  with  the 
various  appearances  of  the  fundus  reflex  by  viewing  it 
from  different  distances,  with  different  lenses  before  the 
eye,  with  different  mirrors,  and  later,  if  he  chooses,  in  a 
number  of  different  eyes  ;  and  all  this  without  concern- 
ing himself  as  to  the  state  of  their  refraction,  or  the  especial 
significance  of  what  he  does.  That  is,  he  should  learn  to 
some  extent  what  are  the  variations  in  the  pupillary  reflex, 
a  few  of  which  are  illustrated  on  the  following  pages, 
before  attempting  to  appreciate  their  significance. 

Without  a  good  understanding  too  of  the  simple  optical 
principles  underlying  the  test,  it  must  remain  a  blind 
routine  and  rule  of  thumb  work,  and  can  never  be  of  the 
highest  utility.  To  aid  in  such  an  understanding  of  them, 
one  may  take  a  strong  (15  D.  to  20  D.)  convex  lens  and  a 
piece  of  card-board  with  a  dot  on  it.  The  lens  can  repre- 
sent the  dioptric  media  of  the  eye,  the  card-board  the  retina, 
and  the  dot  the  light  area  upon  the  retina.  The  card-board 
should  be  held  back  of  the  lens  a  little  farther  than  its 
focal  distance,  and  the  dot  looked  at  through  the  lens  from 
various  distances.  Nearer  the  lens  an  erect  image  of  the 
dot  (blurred  of  course),  and,  farther  away,  an  inverted  image 
will  be  seen,  and  between  the  two  the  phenomena  of  rever- 
sal. The  movement  of  light  on  the  retina  may  be  imitated 
by  a  slight  movement  of  the  card  in  different  directions. 

The  apparent  enlargement  of  the  dot,  as  the  point  of 
reversal  is  approached,  and  the  diminution  of  its  apparent 
size  as  the  point  of  reversal  is  departed  from,  its  diffusion 
and  indistinctness  near  the  point  of  reversal,  and  its  con- 
centration and  greater  definiteness  away  from  the  point  of 
reversal,  are  to  be  observed.  Such  a  combination  of  dot  and 
lens  will  also  beautifully  exhibit  the  phenomena  of  aberra- 


16  SKIASCOPY. 

tion  [See  Chap.  V]  with  its  central  and  peripheral  areas  of 
differing  movement,  the  one  an  erect  and  the  other  an  inverted 
image.  The  difficulty  of  keeping  the  dot  in  view  when 
the  point  of  reversal  is  approached,  will  illustrate  how 
small  a  portion  of  the  retina  is  visible  from  the  point  of 
reversal  when  the  test  is  applied  to  the  eye.  By  holding 
in  combination  with  the  spherical  lens  a  cylindrical  lens 
of  5  D.,  the  distortions  of  the  fundus  reflex  produced  by 
astigmatism,  and  the  band-like  appearances  it  causes  at 
certain  distances,  should  also  be  studied. 

This  is  not  all  to  be  done  at  a  single  trial,  but  the  lens 
and  card  should  be  kept  at  hand  where  they  can  be  used  to- 
parallel  and  elucidate  the  different  conditions  as  they  arise  in 
studying  the  pupillary  reflex. 

The  study  of  the  appearances  in  the  eye  may  thus  be 
carried  on  :  Take  an  eye,  the  refraction  of  which  is  known, 
and  from  a  distance  that  will  give  an  erect  movement, 
throw  the  light  into  the  eye,  and,  by  the  rotation  of  the 
mirror,  produce  and  study  the  erect  movement.  Then  with 
a  lens  which  it  is  known  will  give  an  inverted  movement,, 
the  inverted  movement  is  to  be  similarly  studied.  Finally 
the  lens,  or  position  of  the  observer  is  to  be  so  varied  as  to 
bring  the  point  of  reversal  to  the  eye,  and  the  appearance 
of  the  pupil  from  this  point  is  also  to  be  studied.  In  these 
studies,  and,  indeed,  throughout  the  whole  course,  the 
student  will  find  it  easier  to  master  and  understand  first 
the  appearances  with  the  plane  mirror. 

If  it  is  possible  to  get  an  eye  free  from  astigmatism  or 
aberration  of  any  notable  degree,  these  earlier  studies  of 
the  appearances  will  be  much  simplified.  After  the  above 
have  become  familiar,  the  phenomena  of  astigmatism  may 
be  studied  by  placing  before  the  same  eye,  a  cylindrical 
lens  of  known  strength.     The  point  of  reversal  with  such 


STUDY  OF  THE  TEST.  17 

a  lens  will  give  an  observer  the  appearances  presented  by 
the  pupil  at  this  distance  and  at  the  other  distances  the 
other  appearances  presented  in  astigmatism  can  be  ob- 
tained. 

For  example,  suppose  the  eye  at  the  student's  disposal  is 
hyperopic  i  D.  Let  him  first  place  before  it  the  convex 
2  D  lens.  This  will  bring  the  point  of  reversal  one  metre 
from  the  eye.  With  the  plane  mirror,  let  him  first  study 
the  erect  movement  at  one-half  metre ;  then  study  the 
inverted  movement  at  a  distance  of  two  metres ;  then 
observe  the  eye  from  the  point  of  reversal  at  one  metre,  and 
then  vary  his  distance  so  as  to  study  it  from  intermediate 
points. 

When  he  takes  iip  the  study  of  astigmatism,  he  should 
place  before  such  an  eye,  a  convex  cylindrical  lens  of  2D 
in  addition  to  the  spherical.  Then  from  the  distance  of 
one-third  of  a  metre  he  will  be  able  to  observe  the  band  of 
light  at  right  angles  to  the  axis  of  the  lens,  from  a  distance 
of  one  metre  the  band  of  light  running  in  the  direction  of 
the  axis  of  the  lens,  and  from  other  distances  the  other 
appearances  indicative  of  astigmatism. 

Familiarity  with  the  many  appearances  due  to  aberra- 
tion and  irregular  astigmatism  is  only  to  be  obtained  by 
study  of  eyes  presenting  those  defects.  But,  as  the  great 
majority  of  eyes  present  them  in  notable  degree,  material 
for  such  a  study  is  not  difficult  to  obtain.  Careful  observa- 
tions of  the  corresponding  appearances,  with  the  lens  and 
card-board  already  referred  to,  will  enable  the  beginner 
promptly  to  recognize  the  appearances  of  aberration.  And, 
when  once  he  has  found  an  eye  that  presents  them,  let  him 
observe  them  with  different  lenses,  and  [with  the  plane 
mirror]  from  varying  distances. 

A  considerable  part  of  the  study  of  skiascopy  and  espe- 
2 


18 


SKIASCOPY. 


cially  of  the  appearances  of  positive  aberration  can  be 
carried  on  with  the  aid  of  an^artificial,  schematic,  or  model 
eye.  That  of  Frost  is  one  of  the  best,  although  any,  even 
the  rudest,  will  answer.  In  the  studies  on  the  human  eye, 
it  is  better  to  study  one  eye  long  and  repeatedly,  or  at  most 
to  confine  the  earlier  observations  to  a  few  eyes  than  to 
attempt  to  employ  a  large  number.  Each  additional  eye 
will  introduce  variations  in  the  appearances  presented, 
which  will  at  first  be  onK\,puzzling  and  retard,  rather  than 
assist,  the  mastery  of  the  test. 


CHAPTER  II. 

GENERAL  OPTICAL   PRINCIPLES.      MYOPIA,    EMMETROPIA, 
HYPEROPIA. 

Skiascopy  is  a  method  of  measuring  myopia,  either  the 
myopia  originally  present  in  the  eye  or  that  produced  by  a 
lens  of  known  strength  for  the  purpose  of  measurement. 
In  myopia,  we  have  the  retina  situated  back  of  the  princi- 
pal focus  of  the  dioptric  media,  so  that  the  rays  of  a  certain 
divergence,  that  is  coming  from  a  point  a  certain  finite  dis- 
tance in  front  of  the  eye,  are  brought  to  a  focus  upon  the- 
retina.      Conversely,  the  rays  coming  from  a  point  on  the 
retina  and  passing  out  through  the   crystalline  lens  and 
cornea,  are  brought  to  a  focus  at  the  same  distance  in  front 
of  the  eye.     The  point  for  which  the  eye  is  focused,  and 
the  point  on   the   retina,   on  which  the  focused  rays  are: 
received,  have  the  relation  of  conjugate  foci  to  the  refract- 
ive surfaces  of  the  eye. 

The  Reversal  of  Movement. — The  amount  of  m3'opia 
is  known  when  we  know  the  distance  of  the  point  in  front 
of  the  eye,  which  has  this  relation  of  a  focus  conjugate  to 
the  retina.  Skiascopy  furnishes  a  method  of  determining 
the  position  of  this  point.  Closer  to  the  eye,  than  this 
point  for  which  it  is  focused,  the  observer  may  see  an  erect 
image  of  the  fundus.  Farther  from  the  eye  than  this  point, 
he  can  perceive  an  inverted  image.  Skiascopy  is  a  means 
of  determining  when  the  image  seen  is  erect  and  when  it 
is  inverted,  or  when  it  passes  from  the  erect  to  the  inverted^ 

(19) 


20  GENERAL  OPTICAL  PRINCIPLES. 

This  may  be  understood  from  a  study  of  figure  i. 
Let  M  represent  a  myopic  eye,  A  and  B  being  two  points 
of  the  retina  from  which  rays  emerge  to  reach  the  ob- 
server's eye ;  and  C  and  D  the  points  at  which  these  rays 
coming  from  the  retina  are  focused,  the  rays  coming  from 
A  being  focused  at  C  and  those  from  B  at  D. 

The  apparent  position  of  a  point  is  determined  by  the 
direction  of  a  ray  coming  from  that  point  and  passing 
through  the  nodal  point  of  the  observer's  eye.  Suppose 
the  observer's  eye  is  placed  at  N,  closer  than  the  point  for 
which  the  observed  eye  is  focused.  The  apparent  position 
of  the  point  A  is  determined  b}'  a  ray  which  passes  through 
the  upper  part  of  the  pupil  and  is  turned  down.     It  appears 


Fig.  I. 

in  the  direction  of  a.  The  apparent  position  of  the  point 
B  will  be  located  by  the  ray  coming  through  the  lower 
part  of  the  pupil  and  turned  up.  It  will  be  seen  in  the 
direction  of  b.  Thus,  from  this  position  N,  the  point  A, 
which  is  really  above  appears  above,  and  the  point  B, 
which  is  really  below  appears  below.  The  observer  sees  an 
erect  image. 

When,  however,  the  observer  places  his  eye  at  N',  at 
a  greater  distance  than  that  for  which  the  eye  is  focused, 
the  ray  which  reaches  his  nodal  point  from  A,  will  be 
one  that  comes  through  the  lower  part  of  the  pupil  and  is 
turned  up  ;  so  that  A  will  appear  to  be  located  in  the 
direction  of  a'  in  the  lower  part  of  the  pupil.      From  this 


REVERSAL  OF  MOVEMENT.  21 

position  he  will  judge  the  location  of  B  by  the  ray  which 
comes  through  the  upper  part  of  the  pupil  and  is  turned 
down,  so  that  B  will  appear  to  be  located  in  the  direction 
of  b'  in  the  upper  part  of  the  pupil.  That  is,  the  point  A, 
which  is  really  above,  will  appear  to  be  below,  and  the 
point  B,  which  is  really  below  will  appear  to  be  above. 
The  image  observed  is  inverted. 

The  Point  of  Reversal. — It  is  evident  that  this  change 
in  the  relation  of  the  rays  that  brings  about  the  change  in 
the  apparent  position  of  A  and  B  occurs  at  the  distance  of 
the  points  C  and  D,  at  which,  the  rays  coming  from  the 
retina  are  focused.  Here  it  is  that  these  rays  intersect  and 
take  their  new  relation  which  gives  the  reversal  of  the 
apparent  position  of  the  points  of  the  retina  from  which 
they  come. 

It  is,  therefore,  convenient  in  connection  with  skiascopy 
to  designate  this  point  as  the  point  of  reversal.  This  name 
indicates  the  significance  of  this  point  with  reference  to 
this  test.  Of  course,  it  is  really  the  same  point  as  the  far 
point  of  the  myopic  eye — the  point  for  which  the  eye  is 
focused — the  conjugate  focus  of  the  retina — these  latter 
names  indicating  the  relations  of  the  same  point  in  other 
matters. 

It  is  only  when  the  rays  leave  the  eye,  convergent  only 
when  the  eye  is  myopic,  that  they  ever  come  to  a  focus  in 
front  of  it.  If  the  eye  be  emmetropic  or  hyperopic,  the 
rays  emerging  parallel  or  divergent  remain  so  at  all  dis- 
tances. Hence,  in  emmetropia  and  hyperopia,  there  can  be 
no  point  of  reversal.  From  whatever  distance  the  eye  is 
viewed,  the  image  perceived  is  erect. 

In  myopia,  the  distance  of  the  point  of  reversal  from 
the  eye  depends  on  the  degree  of  convergence  of  the  rays 
as  they  leave  the  cornea — depends  on  the  amount  of  myo- 


22  •  GENERAL  OPTICAL  PRINCIPLES. 

pia.  The  distance  of  the  point  of  reversal  from  the  eye 
being  the  distance  from  the  eye  to  its  far  point  is  the  focal 
distance  of  the  lens  required  to  correct  the  myopia.  So 
that  to  ascertain  the  amount  of  myopia,  we  have  only  to 
determine  the  point  of  re\'ersal  and  then  measure  its  dis- 
tance from  the  eye. 

Skiascopy  determines  the  position  of  the  point  of  re- 
versal by  observation  of  the  direction  of  the  movement  of 
light  and  shade  in  the  pupil.  Other  kinds  of  ophthalmo- 
scopic examinations  attempt  the  recognition  of  the  details 
•of  the  fundus  image.  But,  as  the  point  of  reversal  is 
approached,  the  details  of  the  fundus  image  become  indis- 
tinct and  fade  away  entirely,  so  that  the  location  of  the 
point  of  reversal  cannot  be  accurately  determined  by  such 
an  examination.  On  the  other  hand,  when  this  point  has 
been  so  closely  approached  that  the  fundus  details  are  quite 
indistinguishable,  it  still  remains  easy  to  recognize  the 
direction  of  the  movement  of  light. and  shade  in  the  pupil ; 
and,  from  it,  to  deduce  the  erect  or  reversed  character  of 
the  image.  Skiascopy,  therefore,  determines  the  point  of 
reversal,  and  measures  the  degree  of  myopia  with  much 
greater  exactness  than  the  fundus-image  tests. 

In  skiascopy,  we  watch  the  apparent  movement  of  light 
and  shade  in  the  pupil,  due  to  the  real  movement  of  an 
area  of  light  upon  the  retina.  This  area  of  light  is  secured 
by  reflecting  into  the  eye  the  light  from  a  lamp  with  a 
skiascopic  mirror.  This  is  done  in  a  darkened  room,  in 
order  that  the  retina  outside  of  this  light  area  maybe  dark, 
furnishing  a  contrast  for  the  movements  to  bfe  watched. 
The  movement  of  the  light  area  upon  the  darkened  retina 
is  secured  b)'  varying  the  inclination  of  the  mirror — rotating 
it  about  some  axis  passing  through  the  sight  hole.  The 
movement  produced  by  a  certain  change  in  the  position  of 
the  mirror  depends  on  whether  it  is  plane  or  concave. 


MOVEMENT  OF  LIGHT  ON  THE  RETINA.  23 

Real  Movement  of  the  Light  on  the  Retina.  The 
Source  of  Light. — The  lamp  flame,  or  similar  source  of 
light  used  for  the  test,  may  be  called  the  original  source  of 
light,  in  contra-distinction  to  the  reflection  of  it  from  the 
mirror,  which  being  more  immediately  related  to  the  move- 
ment of  the  light  on  the  retina,  we  shall  call  the  immediate 
source  of  light. 

The  Plane  Mirror. — With  the  plane  mirror  the  imme- 
diate source  of  light  is  behind  the  mirror  as  far  as  the 
original  source  of  light  is  in  front  of  it.  The  rays  reflected 
from  the  mirror  enter  the  eye  under  observation  as  though 
they  had  started  from  this  immediate  source.  As  the  mir- 
ror is  rotated,  the  apparent  position  of  the  immediate 
source  of  light  changes  ;  for  this  immediate  source  is  sit- 
uated upon  a  line  drawn  through  the  original  source  per- 
pendicular to  the  surface  of  the  mirror,  and  necessarily 
changes  with  that  perpendicular  as  the  inclination  of  the 
mirror  changes. 

With  the  change  of  position  of  the  immediate  source 
■of  light,  the  rays  coming  from  it  and  falling  upon  the  eye, 
are  made  to  fall  upon  a  new  part  of  retina,  and  thus  the 
inclination  of  the  mirror  causes  the  change  in  the  part  of 
the  retina  that  is  lit  up  by  the  light  reflected  into  the  eye. 


IM 
'a 


1'  \         B 


A 

Fig.  2. 

What  these  changes  are  can  be  better  understood  by  a 

study  of  figure  2.     L  represents  the  position  of  the  lamp 

iiame,  the  original  source  of  light.       When  the  mirror  is 


24  GENERAL  OPTICAL  PRINCIPLES. 

held  in  the  position  A  A,  the  immediate  source  of  light  is 
situated  at  1,  and  light  entering  the  eye  from  that  direction 
falls  upon  the  retina  toward  a.  When,  however,  the  posi- 
tion of  the  mirror  is  changed  to  BB,  the  immediate  source 
of  light  is  changed  to  1',  from  which,  light  falls  upon  the 
retina  towarb  b.  As  the  mirror  is  rotated  from  A  A  to  BB, 
the  position  of  the  immediate  source  of  light  moves  from  1 
to  1',  and,  as  a  consequence,  the  area  of  light  upon  the 
retina  moves  from  a  to  b.  The  light  on  the  retina  then, 
moves  in  the  direction  that  the  mirror  is  made  to  face.  It 
is  said  to  move  with  the  mirror. 

Only  a  portion  of  the  light  reflected  by  the  mirror 
enters  the  eye,  the  remainder  falls  upon  the  face  and  makes 
a  light  area  on  the  face.  One  may  readily  demonstrate  by 
trial  that  this  area  of  light  cast  by  the  mirror  on  the  face 
also  moves  with  the  mirror  under  all  circumstances. 

The  rays  of  light  coming  from  1  and  1'  intersect  at  the 
nodal  point  of  the  eye  ;  and  passing  directly  on  do  not  again 
change  their  relative  position.  Whatever  the  distance  of 
the  retina  from  this  nodal  point,  the  movement  of  the  light 
upon  it  will  be  in  the  same  direction,  so  that  whether  the 
retina  be  at  H.  as  in  hyperopia,  at  E.  as  in  emmetropia,  or 
at  M.  as  in  myopia,  the  real  movement  of  light  upon  it 
from  a  certain  movement  of  the  mirror  is  alwa}'s  in  the 
same  direction. 

Therefore,  with  the  plane  mirror,  the  real  movement  of  the 
area  of  light  on  the  retina  is  with  the  mirror — ivith  the  area  of 
light  on  the  face,  in  all  states  of  refraction.  This  is  true  for 
all  distances  of  the  light  from  the  mirror,  or  of  the  light 
and  mirror  from  the  tested  eye. 

The  Concave  Mirror. — With  the  concave  mirror  as 
used  in  skiascopy,  the  immediate .  source  of  light  is  a  real 
focus  of  the  mirror,  conjugate  to  the  position  of  the  light, 


MOVEMENT  OF  LIGHT  ON  THE  RETINA. 


25 


and  is  usually  situated  between  the  mirror  and  the  eye  to 
be  tested.  The  position  of  this  immediate  source  varies 
with  the  position  of  the  mirror,  moving  in  the  direction 
that  the  mirror  is  made  to  face  and  causing  an  opposite 
movement  in  the  area  of  light  that  falls  from  it  upon  the 
retina. 


B  A 


^-^ 


Fig.  3. 

In  figure  3  L  again  represents  the  original  source 
of  light.  When  the  mirror  is  in  the  position  AA,  the 
light  falling  upon  it  from  L  is  focused  at  1.,  and  the 
little  inverted  image  of  the  lamp  flame  there  formed  is  the 
immediate  source  of  light.  From  it  the  rays  diverge,  some 
to  fall  upon  the  face,  and  those  entering  the  eye  to  fall 
upon  the  retina  toward  a.  When  the  mirror  is  turned  to 
occupy  the  position  BB,  the  light  falling  upon  it  is  focused 
at  1',  which  becomes  the  new  position  of  the  immediate 
source  of  light,  and  from  which  the  ra}-s  entering  the  eye 
fall  upon  the  retina  toward  b.  As  the  mirror  is  rotated 
from  AA  to  BB,  the  immediate  source  of  light  moves  from 
1  to  1'  and  the  light  upon  the  retina  from  a  to  b.  This  will 
be  the  direction  of  its  movement  in  all  states  of  refraction 
whether  the  retina  be  situated  at  H.  as  in  hyperopia,  at  E. 
as  in  emmetropia,  or  at  M.  as  in  myopia.  The  portion  of 
the  light  which  falls  upon  the  face,  however,  and  forms  the 
facial  area,  as  can  be  readily  demonstrated  by  trial,  moves 
in  the  direction  that  the  mirror  is  made  to  face. 


26  GENERAL  OPTICAL  PRINCIPLES. 

We  have  then,  with  the  concave  mirror,  the  real  movement 
of  the  area  of  light  on  the  retina  is  against  the  mirror,  and 
against  the  light  on  the  face,  in  all  states  of  refraction. 

The  above  is  the  movement  that  occurs  with  the  con- 
cave mirror  used  as  in  skiascopy,  so  far  from  the  original 
source  of  light  and  from  the  eye  to  be  tested,  that  the  con- 
jugate focus  of  the  original  source  of  light  falls  in  front  of 
the  eye.  If,  however,  the  original  source  of  light  be 
brought  so  close  to  the  mirror  that  the  rays  from  it  are  not 
rendered  convergent,  but  continue  to  diverge  after  reflec- 
tion, the  immediate  source  of  light  will  be  a  magnified 
image  of  the  lamp  flame,  situated  behind  the  mirror  as  in 
the  case  of  the  plane  mirror ;  and  the  movement  of  the 
retinal  light  area  will  be  precisely  the  same  as  with  the 
plane  mirror.  Again,  if  the  rays  reflected  by  the  mirror 
are  rendered  convergent,  but  the  eye  to  be  tested  is  brought 
so  near  that  they  cannot  come  to  a  focus  in  front  of  its 
nodal  point,  the  light  will  pass  in  as  though  from  an  imme- 
diate source  back  of  the  mirror,  and  the  movement  of  the 
area  of  light  on  the  retina  will  again  be  like  that  with  the 
plane  mirror.  If  the  light  reflected  upon  the  eye  be  con- 
vergent so  as  to  be  focused  just  at  its  nodal  point,  no  move- 
ment of  light  on  the  retina  such  as  we  have  been  consider- 
ing will  occur,  but  whatever  direction  the  mirror  is  turned, 
so  long  as  the  light  enters  the  eye,  the  retinal  light  area 
remains  stationary. 

It  is  to  be  borne  clearly  in  mind  that  the  movement  so 
far  spoken  of  is  the  real  movement  of  the  area  of  light  upon 
the  retina  as  it  would  appear  from  within  the  eye  itself,  or 
when  viewed  from  behind  the  retina  with  the  sclera  and 
choroid  cut  away. 

The  Apparent  Movement  of  the  Light  in  the  Pupil. 
— What  we  observe  in  skiascopy,  however,  is  the  apparent 


APPARENT  MOVEMENT  IN  THE  PUPIL.  27 

movement  of  the  light  in  the  pupil  as  viewed  from  the 
position  of  the  observer  some  distance  in  front  of  the  eye. 
When  an  erect  image  of  the  retina  is  viewed,  this  apparent 
movement  of  the  light  will  be  in  the  same  direction  as  the 
real  movement.  When  an  inverted  image  is  viewed,  the 
apparent  movement  will  be  in  the  direction  opposite  to  that 
of  the  real  movement. 

The  observer  can  always  watch  the  movement  of  the 
light  area  on  the  face,  and  know  that  with  the  plane  mir- 
ror the  light  area  on  the  retina  always  has  a  real  movement 
in  the  same  direction,  and  with  the  concave  mirror  it  always 
has  a  real  movement  in  the  opposite  direction ;  and  he  has 
only  to  compare  the  apparent  movement  of  the  light  which 
lie  watches  in  the  pupil  with  the  known  direction  of  the 
real  movement  on  the  retina,  to  determine  whether  he  sees 
an  erect  or  an  inverted  image,  When  the  apparent  and 
real  movements  are  in  the  same  direction,  he  knows  (page 
19)  he  is  looking  at  the  eye  from  a  distance  shorter  than 
that  for  which  it  is  focused.  When  the  apparent  and  real 
movements  are  in  opposite  directions,  he  knows  that  he  is 
looking  at  the  eye  from  a  distance  greater  than  that  for 
which  it  is  focused. 

The  direction  of  the  apparent  movement  of  the  light 
then,  will  be  with  the  light  on  the  face  in  hyperopia  and 
in  emmetropia  at  all  distances,  and  in  myopia  when  the 
eye  is  viewed  from  a  point  nearer  than  its  point  of  reversal, 
and  the  apparent  movement  in  the  pupil  will  be  the  opposite 
of  the  real  movement  only  in  cases  of  myopia  when  the 
eye  is  viewed  from  somewhere  beyond  its  point  of  reversal. 

With  the  plane  mirror,  the  apparent  movement  iv'dl  be  with 
the  light  on  the  J  ace  in  hyperopia,  emmetropia,  and  myopia  with 
the  point  of  reversal  behind  the  observer;  and  against  the  light 
on  the  face  in  myopia  viewed  from  beyond  the  iioint  of  reversal. 


28  GENERAL  OPTICAL  PRINCIPLES. 

With  the  concave  mirror  the  apparent  movement  is  against 
the  light  on  the  face  in  hyperopia,  emmetropia,  and  myopia  with 
the  point  of  reversal  behind  the  observer ;  and  is  with  the  light 
on  the  face  only  in  myopia  viewed  from  beyond  the  point  of 
reversal.  This  statement  made  to  conform  to  the  practice 
customary  in  the  use  of  the  concave  mirror  where  the 
observer  keeps  a  constant  distance  of  i  metre  from  the  e}'e 
[corresponding  to  i  D.  of  myopia]  would  be :  the  light 
moves  against  the  light  on  the  face  and  against  the  mirror  in 
hyperopia,  emmetropia  and  myopia  of  less  than  1  D.,  and  only 
moves  with  the  light  on  the  face  in  myopia  of  more  than  1  D. 

These  statements  are  made  with  reference  to  the 
apparent  movement  of  the  light  before  the  state  of  refrac- 
tion has  been  modified  by  any  glass  placed  before  the  eye 
for  that  purpose.  But  they  hold  equally  as  to  hyperopia, 
emmetropia,  or  myoj^ia  remaining  imcorrected,  or  produced 
by  a  lens  placed  before  the  eye.  For  instance  : — In  myopia 
the  movement  remains  against  the  light  on  the  face  with 
the  plane  mirror,  or  with  the  light  on  the  face  with  the 
concave  mirror,  so  long  as  the  concave  lens  employed  is 
not  strong  enough  to  bring  the  point  of  reversal  to  the  dis- 
tance of  the  observer's  eye.  In  hyperopia  or  emmetropia, 
where  the  movement  is  watched  through  a  convex  lens, 
the  movement  remains  with  the  light  on  the  face  for  the 
plane  mirror,  and  against  the  light  on  the  face  for  the  con- 
cave mirror,  until  a  convex  lens  is  used  strong  enough  to 
over-correct  the  hyperopia  and  cause  enough  myopia  to 
bring  the  point  of  reversal  nearer  to  the  eye  than  the  posi- 
tion of  the  observer. 

Rapidity  of  Movement  of  the  Light  on  the  Retina. 
— The  rapidity  with  which  the  light  and  shadow  appear  to 
move  across  the  pupil  depends  first,  on  the  rapidity  of  the 
real  movement    of  the   light    area   upon  the  retina ;    and, 


RAPIDITY  OF  MOVEMENT.  29 

second,  upon  the  magnification  of  the  retina.  The  rapidity 
of  the  real  movement  on  the  retina  depends  : 

On  the  rate  of  movement  of  the  mirror  in  the  observ- 
er's hand. 

On  the  distance  of  the  mirror  from  the  observed  eye. 

On  the  distance  of  the  original  source  of  light  from 
the  mirror. 

And  upon  the  distance  of  the  retina  from  the  nodal 
point  of  the  eye. 

The  rate  of  movement  of  the  mirror  and  the  distance 
of  the  light  from  the  mirror  determine  the  rapidity  of  the 
movement  of  the  immediate  source  of  light  ;  this  being 
greater  as  the  mirror  is  moved  more  quickly,  or  as  the  ori- 
ginal source  of  light  is  more  distant  from  the  mirror.  The 
excursion  which  the  immediate  source  of  light  can  make 
is  limited  by  the  width  of  the  mirror,  and  the  extent  of 
movement  of  the  light  area  on  the  retina  produced  by  the 
movement  of  the  immediate  source  of  light  entirely  across 
the  mirror  depends  on  the  relative  distance  of  the  mirror 
and  the  retina  from  the  nodal  point  of  the  eye.  The  wider 
the  mirror,  or,  the  nearer  it  is  to  the  nodal  point  of  the  eye, 
or  the  farther  the  retina  is  from  that  nodal  point,  the 
greater  the  extent  of  movement  produced  in  the  retinal 
area  of  light  by  a  given  movement  of  the  mirror.  On 
account  of  the  relative  distances  of  the  retina  from  the 
nodal  point,  the  extent  of  the  movement  of  the  light  on 
the  retina  is,  other  things  being  equal,  least  in  the  highest 
hyperopia  and  greatest  in  the  highest  myopia. 

The  rapidity  of  the  real  movement  of  the  light  on  the 
retina  then,  is  increased  : 

By  moving  the  mirror  faster. 

By  carrN'ing  the  original  source  of  light  farther  from 
the  mirror. 


30  GENERAL  OPTICAL  PRINCIPLES. 

By  bringing  the  mirror  closer  to  the  eye. 

By  elongation  of  the  antero-posterior  axis  of  the  eye- 
ball. 

The  real  movement  of  the  light  upon  the  retina  is 
made  slower : 

By  moving  the  mirror  more  slowly. 

By  bringing  the  original  source  of  light  closer  to  the 
mirror. 

By  carrying  the  mirror  farther  from  the  eye. 

By  shortening  of  the  antero-posterior  axis  of  the  eye- 
ball. 

In  using  the  test,  the  distance  of  the  light  from  the 
mirror  is  practically  constant,  and  the  ordinary  variations 
in  the  antero-posterior  axis  of  the  eye-ball  are  so  slight  as 
to  have  no  appreciable  influence.  So  that  the  rapidity  of 
the  real  movement  of  light  on  the  retina  depends  princi- 
pally on  the  rapidity  of  the  movement  of  the  mirror  and 
the  distance  of  the  mirror  from  the  eye. 

Magnification  of  the  Retina. — In  practice  the  rapidity 
of  the  apparent  movement  of  the  light  in  the  pupil  depends 
far  more  on  the  extent  to  which  the  retina  and  the  real  move- 
ment of  light  upon  it  are  magnified,  than  upon  the  actual 
rate  of  that  real  movement.  The  retina  as  viewed  through 
the  pupil  from  different  distances,  is  seen  under  different 
degrees  of  magnification.  When  the  observer's  eye  is 
placed  at  the  point  of  reversal,  the  rays  from  a  single  point 
of  the  retina,  passing  through  all  parts  of  the  pupil,  con- 
verge to  the  observer's  nodal  point,  so  that  the  one  point  of 
the  retina  appears  to  occupy  the  whole  of  the  pupil,  and 
the  retina  is  seen  indefinitely  magnified.  As  the  observer's 
eye  departs  from  the  point  of  reversal,  it  receives  the  rays 
from  an  increasing  area  of  the  retina,  more  and  more  of 
the  retinal  image  occupies  the  same  space  of  the  pupil  and 


MAGNIFICATION  OF  THE  RETINA.  31 

the  retina  is  seen  less  magnified. 

This  is  ilhistrated  in  figure  4,  which  represents  an 
eye  with  its  point  of  reversal  at  A.  If  the  observer's  eye 
be  placed  at  A  it  receives  rays  only  from  the  point  a,  and 
this  point  appears  to  occupy  the  whole  pupil.  If,  however, 
the  observer's  eye  be  placed  at  B,  from  which  rays  would 
be  focused  at  b  behind  the  retina,  and,  at  which,  rays  from 
b  would  be  focused,  the  observer  will  be  able  to  see  in 
the  space  of  the  pupil  all  of  the  retina,  m  n,  included 
between  the  broken  lines  passing  from  B  to  b — all  of  the 
retina,  which  would  receive  a  circle  of  diffusion  if  the  rays 
were  coming  from  the  point  B,     Or,  again,  if  the  observer's 


Fig.  4. 

eye  be  placed  at  C,  from  which  rays  will  be  focused  at  c  in 
front  of  the  retina,  and,  at  which,  rays  coming  from  c  would 
be  focused,  he  will  be  able  to  perceive  the  portion  of  the 
retina,  m  n  included,  between  the  dotted  lines,  passing 
through  c  and  continued  on  to  the  retina — the  area  upon 
which  would  be  formed  a  circle  of  diffusion  by  rays  coming 
from  the  point  C. 

It  follows  then,  that  the  closer  the  observer's  eye  to 
the  point  of  reversal,  the  more  is  the  real  movement  of 
light  upon  the  retina  magnified,  and,  therefore,  the  swifter 
does  it  appear.  The  farther  the  observer's  e}e  is  removed 
from  the  point  of  reversal,  the  less  is  that  real  movement 
of  the  light  on  the  retina  magnified  ;  and  the  slower  is  the 
apparent  movement  as  watched  in  the  pupil. 


32  GENERAL  OPTICAL  PRINCIPLES. 

And,  as  this  source  of  variation  overcomes  all  other 
sources  of  variation  in  the  rate  of  the  apparent  movement 
of  the  light,  [except  the  rate  of  movement  of  the  mirror, 
which  is  to  a  considerable  extent  under  the  control  of  the 
observer]  the  raj^idlty  of  the  apparent  tnovement  of  light  and 
shade  in  the  pupil  increases  as  the  point  of  reversal  is  approached 
and  diminishes  as  that  point  is  departed  from,  and  constitutes 
a  measure  of  the  degree  of  ametropia  remaining  uncor- 
rected. 

Form  of  the  Light  Area. — The  real  form  of  the  light 
area  on  the  retina,  except  under  certain  conditions  in 
astigmatic  eyes,  will  be  circular.  If  the  light  be  perfectly 
focused  on  the  retina  it  is  circular,  because  that  is  the  form 
of  the  source  of  light  employed  (see  Chapter  III).  If  the 
light  be  not  perfectly  focused  on  the  retina,  the  circular 
pupil  gives  its  form  to  the  resulting  area  of  diffusion. 


Fig.  5.  Fig.  6. 

The  apparent  form  of  the  light  area  as  seen  in  the  pupil 
of  astigmatic  eyes  will  be  discussed  in  Chapters  IV  and  V. 
But  in  eyes  free  from  astigmatism  this  form  varies  with 
departure  of  the  observer's  e}'e  from  the  point  of  reversal. 
If  the  magnification  of  the  retina  is  so  slight  that  all  of  it 
occupied  by  the  light  area  is  visible  in  the  pupil  at  one 
time,  that  area  appears  circular  as  represented  in  figure  5. 
But  when  the  point  of  reversal  is  approached  so  that  the 
magnification  of  the  retina  prevents  all  of  the  retinal  light 


FORM  OF  THE  LIGHT  AREA.  33 

area  from  being  seen  at  one  time,  only  a  portion  of  its  ont- 
line  is  visible  as  an  arc  of  the  greatly  enlarged  circle,  as 
shown  in  figure  6  ;  and  the  nearer  to  the  point  of  reversal 
that  the  observer  comes,  the  nearer  does  the  boundary  be- 
tween light  and  shade  approach  to  a  straight  line.  It  must 
be  borne  in  mind,  however,  thai  this  is  still  part  ot  the 
boundary  of  a  circle,  and  hence  that  different  parts  will  run 
in  all  the  different  directions ;  in  contradistinction  to  the 
band-like  appearance  of  astigmatism,  the  direction  of  which 
always  conforms  to  one  or  the  other  of  the  principal  meri- 
dians (see  pp.  47,  55). 

Brilliancy  of  the  Light  in  the  Pupil. — This  depends 
on  the  illumination  of  the  retinal  light  area  and  the  extent 
to  which  that  area  is  magnified. 

The  illumination  of  the  light  area  on  the  retina 
depends  on  the  brightness  of  the  original  source  of  light 
and  the  accuracy  with  which  the  light  coming  from  it  is 
focused  on  the  retina.  The  brighter  the  source  of  light 
and  the  more  accurately  it  is  focused,  the  brighter  the  illu- 
mination of  the  retina.  The  dimmer  the  light  and  the 
larger  the  circle  of  diffusion  over  which  it  is  dispersed,  the 
more  feeble  the  retinal  illumination. 

As  the  immediate  source  of  light  is  usually  near  the 
mirror  (in  front  for  the  concave,  behind  for  the  plane)) 
when  the  mirror  and  the  observer's  eye  approach  the  point 
of  reversal,  or  the  point  of  reversal  is  brought  to  them  by  a 
change  of  lenses,  the  light  being  more  nearly  focused  on 
the  retina,  the  retinal  illumination  becomes  brighter. 

But,  as  the  point  of  reversal  is  approached,  the  appar- 
ent brightness  of  the  light  area  in  the  pupil  is  diminished 
by  the  increasing  magnification  of  the  retina,  which  causes 
the  light  from  a  smaller  part  of  the  retinal  area  to  occupy 
the  whole  space  of  the  pupil.       Hence  the  brightest  light 


34  GENERAL  OPTICAL  PRINCIPLES. 

reflex  is  never  obtained  at  the  point  of  reversal,  but  usually 
in  practice  at  one  or  two  dioptres  from  the  point  of  reversal, 
its  exact  position  being  dependent  on  the  arrangement  of 
the  source  of  light. 

Finding  the  Point  of  Reversal — The  point  of  reversal 
is  to  be  recognized  only  when  the  observer's  eye  is  in  its 
immediate  neighborhood.  This  may  be  effected  either  by 
varying  the  distance  of  the  observer's  eye  from  the  observed 
eye  until  it  comes  to  the  position  of  the  point  of  reversal, 
or  by  varying  the  position  of  the  point  of  reversal  by 
changes  in  the  lenses  placed  before  the  observed  eye  until 
the  point  of  reversal  comes  to  the  chosen  position  of  the 
observer's  eye.  For  reasons  to  be  stated  in  Chapter  VI,  the 
former  method  is  the  better  when  using  the  plane  mirror, 
and  the  latter  is  to  be  resorted  to  when  the  conca\'e  mirror 
is  employed.  In  any  case,  the  trial  movement  across  the 
pupil  shows  by  the  direction  of  the  movement  whether  a 
point  of  reversal  exists  between  the  observer  and  the 
obser\-ed  eye,  and  the  rapidity  of  movement  shows  approx- 
imately [when  the  observer  has  learned  to  appreciate  its 
significance]  the  extent  of  the  interv^al  between  the  position 
of  the  observer  and  the  point  of  reversal.  If  the  movement 
be  slow,  the  inter\'al  may  amount  to  several  dioptres.  If  it 
be  rapid,  the  interval  is  less. 

Upon  these  data  of  the  direction  and  rapidity  of  the 
movement,  the  surgeon  bases  the  next  step  of  the  test,  the 
selection  and  placing  of  the  lenses  before  the  eye.  This 
being  accomplished,  the  test  is  repeated,  the  movement 
with  the  lens  noted  both  as  to  its  direction  and  rapidity, 
and  the  distance  of  the  observer  from  the  patient,  or  the 
strength  of  the  lens  before  the  observed  eye,  varied  in 
accordance  therewith.  This  process  is  continued  until  the 
obser\'er's  eye  reaches  the  point  of  reversal,  or  the  point  of 


POINT  OF  REVERSAL.  35 

reversal  is  brought  by  the  lens  to  the  observer's  eye.  But 
the  test  should  not  be  regarded  as  completed  until  the 
movement  has  been  repeatedh'  viewed  both  from  within 
and  beyond  the  point  of  reversal,  as  well  as  from  that  point. 
Only  by  this  precaution  of  observing  from  a  slightly  greater 
and  a  slightly  less  distance,  or  with  a  slightly  stronger  or 
slightly  weaker  lens  than  that  which  brings  the  point  of 
reversal  to  the  surgeon's  eye,  can  the  certainty  of  a  correct 
result  be  assured. 


CHAPTER  III. 

CONDITIONS    OF   ACCURACY. 

Since  in  skiascopy  one  has  to  observe  the  movement 
of  an  area  of  light  across  the  shaded  retina,  the  size,  bright- 
ness and  sharpness  of  the  contrast  between  the  margin  of 
this  light  area,  and  the  shadow  immediately  adjoining  it 
are  very  important  factors  in  determining  the  definiteness 
and  accuracy  of  the  test.  For  reasons  to  be  presently  dis- 
cussed, the  contrast  between  light  and  shadow  as  seen  in 
the  pupil  necessarily  diminishes  as  the  point  of  reversal  is 
approached.  It  is,  therefore,  important  to  have  the  contrast 
between  light  and  shadow  upon  the  retina  as  sharp  as  pos- 
sible. 

Darkening  the  Room. — To  secure  this  contrast,  the 
retina  outside  of  the  proper  light  area  should  be  in  absolute 
darkness.  This  requires  a  complete  darkening  of  the  room 
in  which  skiascopy  is  practiced,  including  the  shading  of 
the  source  light,  except  in  the  direction  in  which  it  is 
used.  The  difference  in  the  ease  of  the  test  as  applied 
in  a  completely  darkened  room,  as  contrasted  with  its  use 
in  a  partially  darkened  room,  can  only  be  appreciated  by 
one  accustomed  to  applying  it  under  the  former  condition. 

The  Source  of  Light. — To  secure  the  brilliant  illumi- 
nation of  the  light  area,  in  contrast  with  the  complete 
shadow  around  it,  the  source  of  light  must  be  as  bright  as 
possible.  On  account  of  the  difficulty  about  the  sight  hole 
to  be   referred  to  later,   the  arc   electric   light  cannot  be 

(36) 


THE  SOURCE  OF  LIGHT.  37 

■employed,  except  to  illuminate  a  piece  of  ground  glass  as 
suggested  by  Derby.  The  incandescent  electric  light  is 
not  available  on  account  of  its  form,  so  that  recourse  must 
be  had  to  one  of  the  various  illuminating  flames.  Of  these 
the  paraffin  candle  is  the  most  brilliant.  Next  come  the 
heavy  mineral  oils,  and  gas  flames  reinforced  with  the 
richer  hydro-carbons,  or  used  on  the  Welsbach  mantle,  and 
after  this  the  ordinary  illuminating  gas.  But  a  good  flame 
of  the  latter  furnishes  a  satisfactory  illumination. 

It  is  more  important  whatever  flame  is  used  that  the 
brightest  part  of  it  should  be  employed.  With  all  flames 
there  is  at  the  margin  a  comparatively  gradual  shading 
from  light  to  darkness,  which  interferes  with  the  sharpness 
of  the  boundary  of  the  light  area  on  the  retina.  To  secure 
that  sharp  boundary  as  well  as  to  prevent  the  diffused  illu- 
mination of  the  room,  and  to  limit  the  size  of  the  source  of 
light,  the  flame  should  be  entirely  covered  by  an  opaque 
shade  with  an  aperture  of  the  proper  size  placed  opposite 
the  most  brilliant  part  of  the  flame.  This  gives,  under 
proper  conditions  of  focusing,  a  perfectly  sharp  margin  to 
the  light  area  on  the  retina. 

The  size  of  this  opening  in  the  opaque  screen  deter- 
mining the  size  of  the  original  source  of  light  is  governed 
by  various  conflicting  requirements.  Enough  light  must 
be  available  to  give  a  distinct  area  of  light  upon  the  face, 
as  well  as  to  give  sufficient  illumination  within  the  pupil. 
The  source  of  light  must  be  considerably  larger  than  the 
sight  hole  in  the  mirror.  As  the  mirror  is  rotated,  the 
immediate  source  of  light  appears  to  move  across  it,  and  if 
this  source  were  not  larger  than  the  sight  hole,  it  would, 
at  times,  entirely  disappear  within  that  opening.  At  such 
times,  the  light  area  would  disappear  from  the  retina  and 
from  the  pupil,  causing  delay  and  uncertainty  in  the  test. 


38  CONDITIONS  OP  ACCURACY. 

If,  however,  the  immediate  soiirce  of  light  is  sufficiently 
larger  than  the  sight  hole,  no  such  disappearance  of  the 
light  occurs. 

On  the  other  hand,  the  ease  of  distinguishing  special 
forms  of  the  light  area,  or  the  different  movements  in  the 
different  parts  of  the  pupil  is  proportioned  to  the  smallness 
of  the  source  of  light  used.  The  characteristic  band-like 
appearance  of  astigmatism  is  developed  in  proportion  as 
the  light  area  upon  the  retina  approaches  the  limit  of  a 
mathematical  point. 

The  size  of  the  opening  through  which  light  is  ob- 
tained, is  then  a  compromise  between  the  requirements  of 
light  and  the  size  of  the  sight  hole  on  the  one  hand,  and 
need  to  have  the  retinal  light  area  as  small  as  possible  on 
the  other.  In  practice  the  writer  prefers  an  aperture  five 
millimetres  in  diameter,  but  the  beginner  may  find  one  of 
double  that  diameter  more  satisfactory. 

Focusing  of  the  Light  on  the  Retina. — When  the 
rays  coming  from  the  immediate  source  of  light  are  accu- 
rately focused  upon  the  retina,  the  area  of  retinal  illumina- 
tion will  be  the  smallest  and  brightest,  and  will  have  the 
most  definite  edge.  This  accurate  focusing  is  secured  only 
when  the  immediate  source  of  light  is  situated  at  the  point 
of  reversal.  In  searching  for  the  point  of  reversal,  it  is, 
therefore,  advantageous  to  keep  the  immediate  source  of 
light  as  close  to  the  mirror  as  possible. 

With  the  plane  )nirror  the  immediate  source  of  light 
being  a  reflection  of  the  original  source  as  far  behind  the 
mirror  as  the  immediate  source  is  in  front  of  it,  the  closer 
the  original  source  of  light  can  be  brought  to  the  mirror, 
the  closer  will  its  reflection  be  to  the  observer's  eye,  and  to 
the  point  of  reversal  at  the  critical  moment  when  the 
observer's  eye  reaches  that  point.      The  original  source  of 


FOCUSING  OF  THE  LIGHT  ON  THE  RETINA.  39 

light  then  should  be  kept  as  close  to  the  mirror  as  possible 
being  moveable  to  follow  the  movements  of  the  observer's 
eye  and  the  mirror  when  the  distance  of  these  from  the  eye 
under  observation  is  varied. 

When  the  observer  withdraws  to  the  distance  of  two 
metres  or  more  from  the  patient,  it  may  not  be  practicable 
to  keep  the  light  very  close  to  the  mirror,  but  at  such  a 
distance,  the  separation  of  the  source  of  light  from  the 
mirror  becomes  of  small  importance.  For,  if  the  original 
and  immediate  sources  of  light  were  at  the  mirror,  the  rays 
from  the  latter  would  have  a  divergence  of  one-half  D. 
when  they  reached  the  eye  ;  and,  if  the  original  source  of 
light  were  a  metre  in  front  of  the  mirror,  so  that  the  imme- 
diate source  would  be  one  metre  behind  the  mirror,  that  is 
three  metres  from  the  eye,  the  rays  from  it  would  reach 
the  eye  one-third  D.  divergent,  and  the  difference  between 
the  one-half  and  the  one-third  D.  is  so  trifling  as  to  be  of 
no  practical  importance. 

On  the  other  hand,  when  the  surgeon  approaches  close 
to  the  patient's  face,  the  slight  distance  that  must  necessa- 
rily remain  between  the  original  source  of  light  and  the 
mirror  becomes  a  source  of  imperfect  focusing  of  the  light 
on  the  retina ;  and,  therefore,  of  inexactness  in  the  deter- 
mination of  the  point  of  reversal.  Suppose  the  mirror  to 
be  at  five  inches  from  the  eye  and  the  original  source  of 
light  three  inches  from  it,  this  will  make  the  immediate 
source  of  light  eight  inches  from  the  eye,  and  the  ra}'s  from 
it  will  reach  the  pupil  5  D.  divergent  when  the  surgeon  is 
seeking  the  point  of  reversal  corresponding  to  8  D.  of  myo- 
pia. This  difference  of  3  D.  interferes  greatly  with  the 
delicacy  of  the  test. 

With  the  concave  mirror,  the  immediate  source  of  light 
being  a  real  image  of  the  original  source  in  front  of  the 


40  CONDITIONS  OF  ACCURACY. 

mirror,  cannot  be  brought  closer  to  the  mirror  than  its 
principal  focal  distance.  It  is  brought  closest  by  carrying 
the  original  source  of  light  as  far  away  from  the  mirror  as 
possible.  The  original  source  of  light  then,  for  the  con- 
cave mirror,  should  be  behind  the  patient  as  far  as  possi- 
ble. 

Position  of  Greatest  Accuracy. — With  the  plane  mir- 
ror the  immediate  source  of  light  is  necessarily  behind  the 
mirror.  It  will,  therefore,  be  exactly  at  the  point  of  rever- 
sal when  the  mirror  and  the  observer's  eye  are  slightly 
within  the  point  of  reversal.  Hence  the  conditions  of 
accuracy  are  better  complied  with  for  the  observation  that 
is  made  from  within  the  point  of  reversal,  where  the  light 
still  moves  in  the  pupil  with  the  light  on  the  face,  than  for 
the  obser\'ation  that  is  made  from  beyond  the  point  of 
reversal,  where  movement  is  inverted.  The  point  of  rever- 
sal is  then,  with  the  plane  mirror,  most  closely  approxima- 
ted from  the  side  toward  the  observed  eye  ;  and  in  practice 
the  greatest  accurrcy  is  attained  by  considering  the  point 
of  reversal  as  located  at  the  greatest  distance  from  the  eye 
at  which  erect  movement  can  be  seen  in  the  visual  zone  of 
the  pupil. 

With  the  concave  mirror  the  immediate  source  of  light 
being  necessarily  in  front  of  the  mirror,  can  be  brought 
accurately  to  the  point  of  reversal  only  when  that  point  of 
reversal  is  the  focal  distance  of  the  mirror  in  front  of  the 
observ^er's  eye.  It  is  approached  more  accurately  by  the 
lens  which  still  leaves  it  in  front  of  the  observer's  eye, 
than  by  the  lens  which  removes  it  back  of  the  observer's 
eye.  Hence,  with  the  concave  mirror,  the  strongest  con- 
cave lens,  or  the  weakest  convex,  which  allows  the  move- 
ment of  light  in  the  pupil  with  the  light  on  the  face,  is 
the  lens  which  brings  the  point  of  reversal  most  accurately 
to  the  distance  chosen. 


POSITION  OF  GREATEST  ACCURACY.  41 

In  regular  astigmatism,  as  will  be  indicated  in  the 
■chapter  (IV)  upon  that  subject,  the  arrangement  of  the 
light  must  be  modified,  when  it  is  desired  to  develop  the 
band-like  appearance  characteristic  of  that  condition.  For 
the  measurement  of  refraction  in  either  of  the  principal 
meridians,  the  adjustment  of  the  light  should  be  precisely 
the  same  as  for  simple  hyperopia  or  myopia.  But  the  band- 
like appearance  cannot  certainly  be  recognized  unless  the 
necessary  conditions  as  to  the  position  of  the  observ'er  and 
source  of  light  are  carefully  observed.  When  the  proper 
precautions  are  taken  one  can  get  a  characteristic  band  of 
light  with  even  one-half  dioptre  of  astigmatism,  and  by 
that  band  can  fix  the  direction  of  the  principal  meridians 
with  great  accuracy. 

In  the  higher  degrees  of  regular  astigmatism  there  is 
•considerable  difficult}^  in  measuring  the  refraction  of  the 
principal  meridians  with  accuracy.  It  is,  therefore  best, 
before  regarding  the  skiascopic  test  as  completed  to  place 
before  the  eye  such  a  cylindrical  lens  as  appears  to  be 
required  to  correct  the  astigmatism,  repeat  the  test,  and  so 
ascertain  whether  the  astigmatism  has  been  accurately  cor- 
rected. Fuller  references  to  this  matter  will  be  found  in 
the  Chapters  VI  and  VII. 

Irregularities  of  the  Media  and  Surfaces. — These  in- 
terfere with  skiascopy  not  only  by  changes  in  the  apparent 
movement  of  the  light  as  watched  in  the  pupil,  but  also  by 
preventing  the  perfect  focusing  of  the  light  which  falls 
upon  the  retina,  and  in  this  way,  they  limit  to  some  extent, 
the  accuracy  of  the  test,  since  they  are  present  in  some 
degree  in  nearly  all  eyes. 

In  the  case  of  positive  aberration  (see  Chapter  V),  the 
interference  with  focusing  is  of  the  same  kind  as  the  defect 
in  the  refraction  of  a  strong  convex  spherical  lens.      If  one 


42  CONDITIONS  OP  ACCURACY, 

takes  such  a  lens  and  intersects  the  narrowing  pencil  of 
rays  that  have  passed  through  the  lens  with  a  piece  of  card 
board,  he  will  find  that  the  strong  refraction  at  the  margin 
of  the  lens  causes  a  ring  of  condensation  at  the  periphery 
of  the  circle  of  diffusion.  This  ring  is  exhibited  from 
close  behind  the  lens  back  to  its  principal  focus,  beyond 
which,  we  have  the  condensation  at  the  centre  of  the  light 
area  and  a  gradual  fading  away  of  light  around  it.  Hence, 
the  circle  of  diffusion  in  front  of  the  principal  focus  pre- 
sents a  brilliantly  illuminated  edge  in  sharp  contrast  with 
the  shadow  around  it,  while  at  the  principal  focus  and  be- 
hind it,  the  light  area  has  a  sharply  defined  edge,  but  fades 
gradually  into  the  shadow  around  it. 

Therefore,  in  making  the  test,  the  influence  of  posi- 
tive aberration  upon  the  distribution  of  light  in  the  light 
area  is  to  be  utilized  by  having  the  light  focused  not  exactly 
on  the  retina,  but  slightly  back  of  it.  This  may  be  brought 
about  by  having  the  immediate  source  of  light  closer  to 
the  eye  than  the  obseryer's  eye  or  the  point  of  reversal  ; 
conditions  that  are  secured  in  the  use  of  the  concave  mir- 
ror. Hence,  for  positive  aberration  of  a  certain  distribu- 
tion in  the  pupil,  a  sharper  and  more  definitely  bounded 
lieht  area  is  to  be  obtained  bv  the  use  of  the  concave  mir- 
ror  than  can  be  had  with  the  plane  mirror. 

With  negative  aberration,  where  the  refraction  is 
weaker  near  the  peripher}^  of  the  pupil,  the  condensation 
ring  of  light  is  less  pronounced  and  is  found  back  of  the 
principal  focus  for  the  central  visual  area.  For  this  form 
of  aberration  the  plane  mirror  enabling  the  observer  by 
pushing  the  source  of  light  from  the  mirror  to  get  the  light 
focused  in  front  of  the  retina  has  some  advantage  over  the 
concave  mirror. 

The  interference  with  the  focusing  of  the  light  on  the 


IRREGULARITIES  OF  THE  MEDIA  AND  SURFACES.  43 

retina  due  to  irregular  astigmatism  cannot  be  overcome  in 
any  way,  and  it  impairs  the  value  of  the  test  and  makes  it 
more  difficult  to  apply  in  eyes  presenting  marked  defects 
of  this  kind. 

Distance  of  the  Surgeon  from  the  Patient It  will 

always  be  impossible  to  determine  the  point  of  reversal 
with  perfect  exactness.  The  best  that  can  be  done  is  to 
make  out  that  it  lies,  within  narrow  limits  of  possible 
error,  at  about  a  certain  distance.  It  may  be  an  inch  or 
two  nearer,  it  may  be  an  inch  or  two  farther  off. 

If  the  distance  be  a  short  one,  if  the  lens  used  is  such 
that  the  point  of  reversal  is  brought  close  to  the  observed 
eye,  the  possible  inaccuracy  of  distance  will  cause  an 
appreciable  error  in  estimating  the  refraction  measured  in 
dioptres.  For  instance :  at  eight  inches  from  the  eye,  two 
inches  additional,  making  ten  inches,  will  mean  a  whole 
dioptre  of  refraction,  and  two  inches  less,  making  the  dis- 
tance six  inches,  will  mean  a  difference  of  a  dioptre  and  a 
half.  On  the  other  hand,  at  eighty  inches,  a  foot  either 
way  will  correspond  to  less  than  one-quarter  of  a  dioptre  of 
inexactness.  Hence,  for  accurate  work  it  is  best  to  make 
the  determination  of  the  point  of  reversal  at  the  greatest 
distance  at  which  it  can  be  certainly  made  in  the  visual 
zone.  The  importance  of  this  has  been  especially  dwelt 
upon  by  Randall  (Trans.  Section  on  OpJdhalmoL  Am  Med. 
Assoc,  1894,  p.  63). 

What  this  distance  may  be  will  var}-  in  different  eyes. 
In  general,  it  is  limited  by  the  size  of  the  area  in  which 
the  movement  of  light  and  shade  is  to  be  watched.  The 
pupil  fully  dilated  may  be  eight  or  ten  millimetres  in 
diameter,  and  movement  across  the  whole  width  of  such  a 
pupil  could  be  readily  watched  at  a  distance  of  4  to  6 
metres.     But  the  diameter  of  the  visual  zone  of  the  pupil, 


44  CONDITIONS  OF  ACCURACY. 

the  only  area  in  which  the  movement  is  of  practical 
importance,  is  commonly  much  less  than  this,  say  from  4 
to  6  millimetres,  and  the  movement  of  light  across  it  can 
only  be  satisfactorily  studied  within  the  distance  of  two  or 
three  metres. 

Beyond  one  metre,  however,  the  necessary  inaccuracies 
of  distance  become  usually  of  slight  practical  importance. 
In  cases  of  aberration  invading  the  central  portions  of  the 
pupil  and  still  more  in  cases  of  irregular  astigmatism,  the 
visual  zone  is  considerably  less  in  area  than  in  the  ordinary 
normal  eye.  In  these  cases,  the  test  must  be  applied  from 
a  still  shorter  distance,  often  one-half  or  one-third  of  a 
metre,  or  even  less. 

With  the  plane  mirror  it  is  easy  to  adopt  any  distance 
that  suits  the  particular  case.  With  the  concave  mirror 
any  considerable  variation  in  the  distance  requires  a  corre- 
sponding variation  in  the  focus  of  the  mirror  used.  A  mir- 
ror of  shorter  focus  being  employed  when  the  distance 
between  the  observer  and  patient  must  be  short ;  and  of 
longer  focus  if  a  greater  distance  is  to  be  maintained. 

The  reason  for  this  is  that  if  the  concave  mirror  be 
brought  too  close  to  the  observed  eye  it  gives  an  immediate 
source  of  light  relatively  too  large,  while  if  it  be  removed 
too  far  from  the  patient's  eye,  the  diffusion  is  so  rapid  that 
it  gives  an  illumination  that  is  too  feeble.  These  changes 
are  much  more  rapid  with  the  concave  than  with  the 
plane  mirror ;  as  one  may  readily  demonstrate  by  holding 
both  mirrors  in  his  hand  in  the  darkened  room  and  reflect- 
ing areas  of  light  upon  a  wall  from  various  distances.  I 
have  elsewhere  {Journal  of  the  Am.  Med.  Assoc,  Sept.  4, 
1886)  demonstrated  the  relations  of  the  one  to  the  other. 
In  general  the  distance  at  which  a  concave  mirror  can  be 


DISTANCE  OF  THE  SURGEON  FROM  THE  PATIENT  45 

used  to  best  advantage  is  a  little  over  four  times  its  focal 
distance. 

For  the  majority  of  cases  then,  a  distance  of  from  5/2 
to  2  metres  is  convenient  for  the  plane  mirror ;  and  one 
metre  or  a  little  less  for  the  concave  mirror,  which  shonld 
have  a  focal  distance  of  from  20  to  25  centimetres. 

When  it  is  desired  to  make  the  shadow  test  as  accurate 
as  possible,  it  is  well  to  complete  the  test  by  placing  before 
each  eye  lenses  representing  its  supposed  correction,  with 
such  addition  to  the  convex  or  diminution  of  the  concave 
spherical  as  shall  bring  the  point  of  reversal  to  the  great- 
est distance  at  which  it  can  be  satisfactorily  studied  in  the 
particular  eyes  in  question  ;  and  then  to  test  the  movement 
of  light  and  shade  at  that  distance,  looking  especially  for 
uncorrected  astigmatism,  and  comparing  the  one  eye  with 
the  other  for  any  evidence  of  remaining  inequality  of 
refraction. 


CHAPTER  IV. 

REGULAR   ASTIGMATISM. 

The  essential  fact  of  regular  astigmatism  is  that  in  two 
different  directions,  at  right  angles  to  each  other  [the  prin- 
cipal meridians],  the  curvature  of  the  dioptric  surfaces  dif- 
fer, so  that  they  exert  unequal  refractive  power ;  and  that 
in  all  other  directions,  or  meridians,  the  refractive  power 
bears  such  a  relation  to  the  refractive  power  of  these  prin- 
cipal meridians,  that  it  is  only  necessary  to  consider  what 
happens  in  their  direction. 

Two  Points  of  Reversal, — In  such  an  eye,  the  rays 
coming  from  the  same  point  of  the  retina,  and  passing  out 
through  surfaces  that  refract  imequally  in  different  merid- 
ians, must  leave  the  eye  with  different  degrees  of  divergence 
or  convergence  in  the  directions  of  these  different  meridians. 
If  the  ra)'s  are  convergent,  or  rendered  so  by  passing  through 
a  convex  spherical  lens,  they  will  be  more  convergent  in 
one  principal  meridian  than  the  other,  and  the  point  of 
reversal  for  one  principal  meridian  will  be  at  a  different 
distance  from  the  eye,  from  the  point  of  reversal  for  the 
other  principal  meridian.  The  position  of  the  point  of 
reversal,  giving  the  amount  of  myopia  (either  original  or 
produced)  in  the  principal  meridian  to  which  it  belongs, 
the  difference  between  the  amounts  of  myopia  in  the  two 
principal  meridians  will  be  the  astigmatism.  The  general 
plan  of  measuring  astigmatism  by  skiascopy,  therefore,  is 
to  ascertain  the  point  of  reversal  and  measure  the  degree 

(46) 


THE  BAND-LIKE  APPEARANCE.  47 

of  myopia  for  each  principal  meridian,  and  by  subtracting 
the  one  from  the  other,  to  find  the  amount  of  regular 
astigmatism. 

The  Band-like  Appearance. — This  difference  in  the 
position  of  the  points  of  reversal  for  the  different  meridians, 
gives  rise  to  certain  phenomena  of  great  practical  import- 
ance in  skiascopy.  It  is  true  of  the  astigmatic  as  of  the 
non-astigmatic  eye,  that,  as  the  point  of  reversal  is  ap- 
proached, the  image  of  the  retina  seen  through  the  pupil 
becomes  magnified  (see  Chapter  II).  And,  it  necessarily 
follows  that  when  the  observer's  eye  is  nearer  to  the  point 
of  reversal  for  one  meridian  than  it  is  to  the  point  of  rever- 
sal for  the  other  meridian,  that  the  retinal  image  is  more 
magnified  in  the  direction  of  the  principal  meridian,  to 
which  the  nearer  point  of  reversal  belongs. 

When  the  observer's  eye  is  placed  at  the  point  of  re- 
versal for  one  meridian,  the  retinal  image  becomes  indefi- 
nitely magnified  in  the  direction  of  that  meridian,  while 
comparatively  little  magnified  in  the  direction  at  right 
angles  to  it.  Each  point  of  the  retina  then  appears  in  the 
pupil  as  a  line  running  in  the  direction  of  that  principal 
meridian ,  and  the  retinal  light  area,  which  consists  of  a 
number  of  these  points,  takes  the  form  of  an  elongated  band 
of  light  running  in  the  direction  of  the  principal  meridian, 
which  has  its  point  of  reversal  at  the  observer's  eye.  This 
is  the  band-like  appearance  of  the  light  in  the  pupil,  char- 
acteristic of  astigmatism  bounded  by  the  "  linear  shadow  " 
of  Bowman.  Figure  7  represents  this  appearance  when 
the  eye  is  placed  at  the  point  of  reversal  for  one  principal 
meridian,  represented  about  twenty  degrees  from  the 
vertical  ;  and  figure  8  represents  the  appearance  presented 
at  the  point  of  reversal  for  the  other  principal  meridian, 
twenty  degrees  from  the  horizontal.     Its  direction  is  always 


48 


REGULAR  ASTIGMATISM. 


that  of  the  principal  meridian,  at  whose  point  of  reversal 
it  is  seen,  and  it  is  more  pronounced,  in  proportion  to  the 
degree  of  astigmatism,  the  nearness  of  approach  to  the  point 
of  reversal,  and  the  perfection  of  the  focusing  of  the  light 
upon  the  retina  in  the  direction  perpendicular  to  this  prin- 
cipal meridian,  that  is,  in  the  other  principal  meridian. 


Fig. 


Fig.  8. 


In  estimating  astigmatism  by  skiascopy,  two  distinct 
things  are  to  be  done,  which  require  different  arrangements 
of  the  source  of  light.  The  first  is  to  determine  accurately 
the  direction  of  the  principal  meridians  by  bringing  out 
most  distinctly  this  band-like  appearance  in  the  pupil,  in- 
dicating the  direction  of  one  of  these  principal  meridians ; 
the  other  being  always,  for  regular  astigmatism,  at  right 
angles  thereto.  The  second  thing  to  be  done  is  to  measure 
accurately  the  refraction  in  each  of  these  principal  merid- 
ians, testing  them,  of  course,  one  at  a  time. 

The  test  proceeds  at  first  as  for  myopia  or  hyperopia  in 
a  non-astigmatic  eye,  until  a  point  of  reversal  is  found. 
Then  it  is  discovered  that  this  point  of  reversal  is  only  for 
the  movement  of  light  and  shadow  in  one  direction,  and 
does  not  hold  for  movements  at  right  angles  to  that  direc- 
tion. The  observer  has  now  brought  his  eye  to  one  point 
of  reversal  where  the  band-like  appearance  can  be  best  per- 
ceived. But,  as  he  has  been  working  with  the  original 
source  of  light  in  the  position  most  favorable  for  the  meas- 


THE  BAND-LIKE  APPEARANCE.  49 

urement  of  hyperopia  and  myopia,  the  position  that  brings 
the  immediate  source  of  light  as  close  as  possible  to  the 
mirror  (see  Chapter  III),  he  will  probably  see  very  little  ap- 
pearance of  the  band  in  the  pupil,  even  with  the  higher 
degrees  of  astigmatism.  The  reason  for  this  is  that  with 
the  immediate  source  of  light  in  this  position,  the  light  is 
most  accurately  focused  on  the  retina  in  the  direction  that 
the  band  should  take.  And,  in  the  direction  at  right  angles 
to  the  band,  the  focusing  is  quite  incomplete,  so  that  the 
diffusion  at  what  should  be  the  sides  of  the  band  partly  or 
entirely  neutralizes  the  effect  produced  by  the  magnification 
of  the  retina,  which,  otherwise,  would  cause  the  band-like 
appearance. 

In  order  to  bring  out  this  band-like  appearance,  it  is 
necessary  to  make  the  focusing  from  side  to  side  of  the  band 
as  perfect  as  possible.  And,  to  secure  the  perfect  focusing 
in  the  principal  meridians  at  right  angles  to  the  one  in 
which  the  band  is  sought,  the  immediate  source  of  light 
must  be  brought  to  the  point  of  reversal  for  that  other 
principal  meridian.  The  band-like  appearance  is  most  per- 
fectly developed  luhen  the  observer's  eye  is  at  the  point  of  reversal 
for  one  principal  meridian,  and  the  immediate  source  of  light  at 
the  point  of  reversal  for  the  other  principal  meridian. 


Fig.  9. 


In  figure  9,  the  solid  lines  represent  the  vertical  merid- 
ian of  an  astigmatic  eye  and^  the  rays  emerging,  so  turned 
4 


50  REGULAR  ASTIGMATISM. 

in  that  meridian,  as  to  give  the  point  of  reversal  at  V.  The 
broken  lines  represent  the  less  curved  horizontal  meridian 
of  the  cornea,  and  the  rays  so  turned  in  that  meridian  as  to 
give  a  point  of  reversal  at  H.  The  dotted  lines  represent  a 
plane  mirror,  P  P,  with  the  eye  of  the  observer  at  V,  and 
the  light  h  pushed  off  from  the  mirror,  so  that  the  rays 
enter  the  eye  as  though  they  came  from  H,  and  are  per- 
fectly focused  on  the  retina  in  the  horizontal  meridian,  ren- 
dering most  distinct  the  appearance  of  a  vertical  band. 

For  illustration,  suppose  a  case  (which  the  student  will 
do  well  to  reproduce  for  actual  study,  either  in  the  artificial 
eye  or  by  lenses  placed  before  the  living  eye)  having  com- 
pound myopic  astigmatism,  the  vertical  meridian  of  the 
cornea  being  2  D.  myopic  and  the  horizontal  meridian  i  D. 
myopic.  When,  with  the  plane  mirror,  the  observer's  eye 
is  one-half  metre  from  the  observed  eye,  it  will  be  at  the 
point  of  reversal  for  the  vertical  meridian,  and  in  a  position 
to  see  a  vertical  band  of  light.  But,  if  the  source  of  light 
be  placed  as  close  to  the  mirror  as  possible,  the  rays  from 
it  will  be  the  more  accurately  focused  upon  the  retina  in 
the  vertical  meridian  and  more  diffused  horizontally,  so 
that  the  I'eal  form  of  the  retinal  light  area  will  be  rather 
that  of  a  horizontal  line  or  band. 

Now,  from  the  observer's  position,  the  retina  is  most 
magnified  in  the  vertical  direction,  and  this  vertical  magni- 
fication would  cause  a  point  of  light  on  the  retina  to  appear 
as  a  vertical  band  in  the  pupil  ;  but,  with  the  light  area 
really  in  the  form  of  a  horizontal  band,  the  effect  of  the 
magnification  is  largely  neutralized  and  the  appearance  in 
the  pupil  may  be  quite  indefinite. 

To  bring  out  the  band-like  appearance  :  While  keeping 
the  observer's  eye  and  mirror  in  the  same  position,  the 
the  original  source  of  light  must  be  pushed  off  from  the  mir- 


THE  BAND-LIKE  APPEARANCE.  51 

ror  one-half  metre,  the  immediate  source  then  retreats  cor- 
respondingly behind  the  mirror,  and  approaches  the  posi- 
tion of  the  point  of  reversal  for  the  horizontal  meridian, 
one  metre  from  the  eye. 

With  the  light  and  mirror  in  this  relation  to  the  eye, 
the  rays  are  focused  upon  the  retina  perfectly  in  the  hori- 
zontal meridian  and  diffused  in  the  vertical  meridian,  so 
that  the  real  form  of  the  retinal  area  of  light  is  a  vertical 
line  or  band.  This  vertical  line  or  band  being  viewed  from 
the  point  of  reversal  of  the  vertical  meridian  (where  it  will 
be  greatly  magnified  in  the  vertical  direction  and  but 
slightly  magnified  in  the  horizontal  direction),  gives  rise  to 
the  appearance  of  the  most  distinct  vertical  band  of  light 
in  the  pupil.  And,  under  these  conditions,  the  presence  of 
the  astigmatism  and  the  direction  of  one  of  its  principal 
meridians  is  most  clearly  and  accurately  revealed. 

Taking  the  same  case  and  using  the  concave  mirror  at 
a  distance  of  one  metre,  which  is  the  point  of  reversal  for 
the  horizontal  meridian,  the  appearance  of  a  horizontal 
band  of  light  in  the  pupil  should  be  most  distinctly  visible. 
But,  in  order  to  develop  it  clearly,  it  will  be  needful  to 
bring  the  original  source  of  light  so  near  to  the  mirror  that 
the  immediate  source  will  be  one-half  metre  in  front  of  the 
mirror,  that  is,  one-half  metre  in  front  of  the  observed  e}'e, 
at  the  point  of  reversal  for  the  vertical  meridian.  For  it  is 
from  this  position  the  light  will  be  most  perfectly  focused 
on  the  retina  in  the  vertical  meridian,  while  diffused  in  the 
horizontal  meridian,  and  the  horizontal  magnification  of 
the  retina  at  the  point  of  reversal  for  the  horizontal  merid- 
ian where  the  observer's  eye  is  placed,  will  emphasize  and 
increase  the  appearance  of  the  horizontal  band  of  light 
then  thrown  on  the  retina. 

Since,  with  the  plane  mirror,  the  immediate  source  of 


52  REGULAR  ASTIGMATISM. 

light  is  always  back  of  the  mirror,  and  cannot  be  brought 
in  front  of  it,  the  direction  of  the  band  can  only  be  accu- 
rately determined  for  the  meridian  whose  point  of  reversal 
is  nearest  the  eye.  It  is  only  with  the  eye  and  mirror  at 
this  point  of  reversal  that  one  is  able,  with  the  plane  mir- 
ror, to  bring  the  immediate  source  of  light  to  the  other 
point  of  reversal.  And,  with  the  concave  mirror,  since  the 
immediate  source  of  light  is  always  in  front  of  the  mirror, 
the  band-like  appearance  can  only  be  distinctly  brought  out 
in  the  meridian  which  has  its  point  of  reversal  the  farther 
from  the  eye,  as  only  with  the  eye  at  that  point  of  reversal 
can  the  immediate  source  of  light  with  the  concave  mirror 
be  brought  to  the  other  point  of  reversal. 

With  either  the  plane  or  concave  mirror,  only  the  band 
in  one  of  the  principal  meridians  can  be  most  distinctly 
developed.  But  it  is  unnecessary  in  practice  to  bring  out 
the  bands  in  both  meridians,  since,  by  knowing  the  direc- 
tion of  one  principal  meridian,  the  other  being  always  per- 
pendicular to  it,  is  also  known. 

The  measurement  of  the  refraction  in  either  of  the 
principal  meridians  of  astigmatism,  is  quite  similar  to  the 
measurement  of  refraction  in  hyperopia  and  myopia.  To 
determine  whether  the  movement  of  light  in  the  pupil  in  a 
certain  meridian  is  with  or  against  the  movement  of  light 
upon  the  face,  it  is  necessary  that  the  focusing  of  the  light 
on  the  retina  be  as  perfect  as  possible  in  that  particular 
meridian.  To  secure  this,  the  immediate  source  of  light 
must  be  as  close  as  possible  to  the  position  of  the  observer's 
eye  (see  Chapter  III).  Hence,  having  determined  the  ex- 
istence of  the  astigmatism  and  the  direction  of  its  princi- 
pal meridians,  the  measurement  in  these  meridians  will 
proceed  as  the  measurement  of  myopia  or  hyperopia. 


CHANGES  IN  THE  LIGHT  AREA.  53 

Changes  in  the  Light  Area  at  Different  Distances. — 

In  regular  astigmatism,  supposing  the  eye  to  be  myopic  in 
all  meridians,  or  a  convex  lens  placed  before  it  sufficiently 
strong  to  over-correct  the  hyperopia  in  all  meridians,  the 
observer  using  a  plane  mirror  and  viewing  the  eye  from 
different  distances,  will  be  able  to  recognize  the  following 
changes  in  the  appearance  and  movement  of  the  light  in 
the  pupil. 

From  a  position  within  the  point  of  reversal  of  the 
more  myopic  meridian,  the  light  will  be  seen  to  move  with 
the  light  on  the  face,  in  all  directions.  As  the  observ- 
er's eye  is  withdrawn  from  the  observed  eye,  and  approaches 
the  point  of  reversal  for  the  more  myopic  meridian,  the 
light  area  in  the  pupil  becomes  elongated  in  this  meridian  ; 
and,  while  the  movement  is  still  with  the  light  on  the  face 
in  all  meridians,  it  becomes  more  rapid  in  the  direction  of 
this  elongation  than  in  the  direction  perpendicular  thereto. 

The  observer,  withdrawing  his  eye  still  farther  on 
reaching  the  point  of  reversal  for  the  more  myopic  merid- 
ian, [V,  in  figure  9,]  is  unable  to  distinguish  the  movement 
in  this  meridian,  while  the  movement  in  the  meridian  at 
right  angles  to  it  is  still  with  that  of  the  light  on  the  face. 

This  point  being  reached,  if  the  original  source  of  light 
be  pushed  away  from  the  mirror,  so  that  its  reflection,  the 
immediate  source  of  light  approaches  the  point  of  reversal 
for  the  less  myopic  meridian,  the  form  of  the  light  in  the 
pupil  becomes  a  distinct  band  running  in  the  direction  of 
the  more  myopic  meridian,  readily  seen  to  move  from  side 
to  side,  but  without  perceptible  movement  in  the  direction 
of  its  length. 

Bringing  the  source  of  light  back  to  its  usual  position 
close  to  the  mirror,  and  withdrawing  his  eye  still  farther 
from  the  eye  under  observation,  the  observer  again  sees  the 


54  REGULAR  ASTIGMATISM. 

movement  of  the  light  in  the  pupil  in  all  directions.  But 
in  the  direction  of  the  most  myopic  meridian,  it  is  now 
against  the  light  on  the  face  ;  while  in  the  meridian  at  right 
angles  to  this,  it  is  still  with  the  light  on  the  face.  The 
band-like  appearance  is  now  lost  entirely  ;  the  area  of  light 
in  the  jDupil  taking  at  one  distance  the  same  shape  as 
though  no  regular  astigmatism  were  present. 

But,  as  the  point  of  reversal  for  the  less  myopic  merid- 
ian is  approached,  elongation  in  the  direction  of  that  me- 
ridian may  be  noticed,  and  the  erect  movement  of  the  light 
in  that  meridian  becomes  more  rapid  than  the  inverted 
movement  now  seen  in  the  more  myopic  meridian.  When 
the  point  of  reversal  for  the  less  myopic  meridian  [H,  figure 
9]  is  reached,  the  movement  in  its  direction  ceases,  but  it 
is  impossible,  at  this  point  (with  the  plane  mirror),  to 
bring  out  so  distinct  a  band  as  was  seen  in  the  direction  of 
the  other  meridian. 

Withdrawing  still  farther,  the  light  in  the  direction  of 
the  less  myopic  meridian  begins  to  have  an  inverted  move- 
ment, at  first  very  rapid  as  compared  with  the  movement 
in  the  more  myopic  meridian  ;  but,  as  the  observer  with- 
draws farther  from  this  second  point  of  reversal,  the  differ- 
ence in  rate  of  movement  in  the  two  meridians  becomes 
less  noticeable. 

With  the  concave  mirror,  the  same  series  of  appearan- 
ces are  present,  except  that  the  directions  of  movement  are 
reversed — "  erect  movement "  meaning  movement  of  the 
light  in  the  pupil  against  the  movement  of  the  light  on  the 
face,  and  against  the  mirror ;  and  "  inverted  mo\'ement " 
meaning  the  movement  of  the  light  in  the  pupil  with  the 
mirror  and  with  the  light  on  the  face.  With  the  concave 
mirror  the  meridian  in  which  it  is  possible  to  bring  out  the 
band-like  appearance  of  the  light  most  distinctly  is  the 


MOVEMENT  OF  THE  BANDS  IN  ASTIGMATISM.  55 

meridian  of  less  myopia  ;  and  it  will  be  necessary  to  bring 
about  the  series  of  changes  in  the  movement  of  the  light 
area,  which  has  been  referred  to,  by  changes  of  the  lens 
placed  before  the  eye,  and  not  by  changes  in  the  observer's 
distance  from  the  eye  studied. 

Direction  and  Movement  of  the  Bands  in  Astigma- 
tism.— The  reason  for  the  constant  conformity  of  the  di- 
rection of  these  bands  of  light  to  the  principal  meridians  of 
refraction  is  obvious  from  their  dependence  on  the  magnifi- 
cation of  the  retina.  That  conformity  sharply  separates 
them  from  the  somewhat  similar  appearance  seen  near  the 
point  of  reversal  in  eyes  free  from  astigmatism  (page  32 ). 
O/^r:;- — ^o  The  apparent  movement  always 

-^^       at  right  angles  to  their  direction  is 
/        dependent  on  an  optical  illusion,  of 
/  which  one  may  satisfy  himself  by 

.'^  making  a  hole  in   the   centre  of  a 

/  sheet  of  paper,  holding  behind  this 

hole  the  edge  of  a  card,  and  moving 
it  in  a  direction  oblique  to  this  edge. 
iG.  10.  rj^-^^  motion  will  appear  to  be  in  a 

direction  nearly  or  quite  perpendicular  to  the  edge  seen. 

Thus,  in  figure  10,  the  real  movement  of  the  card  be- 
hind the  opening,  or  the  band  of  light  behind  the  pupil, 
may  be  in  the  direction  O  o.  But  the  movement  will  appear 
to  be  in  the  direction  P  p. 


CHAPTER  V. 

ABERRATION   AND    IRREGULAR   ASTIGMATISM. 

In  astigmatism,  strictly  regular,  though  the  refraction 
differs  in  different  meridians,  in  any  given  direction  or 
meridian  it  is  the  same  at  all  parts  of  the  pupil.  In  aber- 
ration and  irregular  astigmatism,  the  refraction  differs  in 
different  parts  of  the  pupil,  even  in  the  same  meridian. 
All  eyes  present  variations  of  this  kind,  and  these  varia- 
tions constitute  an  obstacle  to  the  measurement  of  refrac- 
tion b}-  skiascop}'  or  by  any  other  method. 

Appearances  of  Irregular  Astigmatism. — To  the  be- 
ginner with  skiascopy,  they  constitute  the  most  serious 
obstacle  he  has  to  encounter.  For  one  who  has  thoroughly 
mastered  the  principles  of  the  test  and  become  familiar 
with  the  various  appearances  of  light  and  shade  in  the 
pupil,  mistakes  due  to  aberration  or  irregular  astigmatism 
are  readily  avoided,  while  the  reason  for  any  uncertaint}'  as 
to  the  results  obtained  by  other  methods,  or  any  failure  to 
secure  perfect  vision,  on  account  of  these  defects  is  revealed. 

If  we  suppose  two  parts  of  the  pupil,  one  of  which  has 
its  point  of  reversal  at  the  observer's  eye,  while  the  other 
is  at  a  considerable  distance  therefrom,  the  illumination 
of  the  former  will  be  the  more  feeble,  of  the  latter  the  more 
brilliant ;  the  movement  of  the  light  in  the  former,  if  per- 
ceptible, will  be  rapid,  in  the  latter,  slow.  If  one  watches 
two  parts  of  the  pupil,  one  of  which  has  its  point  of  rever- 
sal back  of  the  observer's  eye,  and  the  other  in  front  of  it ; 

(5(5) 


IRREGULAR  ASTIGMATISM.  57 

in    the    former  the    light  will  have  a  direct    and   in    the 
latter  an   inverted  movement. 

With  the  irregular  astigmatism  due  to  preceding  cor- 
neal inflammation,  or  to  the  changes  in  the  refraction  of  the 
lens  that  sometimes  precede  cortical  cataract,  the  pupil  ap- 
pears broken  up  into  a  considerable  number  of  distinct 
areas,  each  of  which  has  its  separate  movement  of  light  and 
shadow,  constituting  the  typical  ophthalmoscopic  or  skia- 
scopic  picture  of  irregular  astigmatism.     The  appearance 


Fig.  II.  Fig.  12. 

caused  by  irregular  astigmatism  following  corneal  disease 
is  shown  in  figure  ii.  That  due  to  changes  in  the  lens- 
such  as  may  precede  cortical  senile  cataract  is  shown  in 
figure  12,  in  which  the  black  lines  represent  fixed  spicules- 
of  actual  opacity,  while  the  other  parts  of  the  pupil  indicate- 
merely  refractive  differences,  and  change  from  light  to  dark 
or  dark  to  light,  as  the  inclination  of  the  mirror  is  varied. 
Some  such  appearance  is  sometimes  presented  by  young- 
persons,  indicating  a  congenital  defect  which  may  not 
noticeably  increase  in  many  years. 

If  the  differences  of  refraction  in  the  different  parts  of 
the  pupil  are  slight — that  is,  if  the  aberration  or  irregular 
astigmatism  is  of  low  degree — these  differences  of  illumina- 
tion and  movement  will  not  be  perceptible  until  the  ob- 
server puts  his  eve  close  to  the  point  of  reversal.  Bfit  at 
5 


58  ABERRATION  AND  IRREGULAR  ASTIGMATISM. 

the  point  of  reversal,  they  become  perceptible  and  consti- 
tute a  striking  phenomenon  in  almost  all  eyes  ;  and,  to  the 
observer  who  does  not  understand  their  significance,  one 
that  is  extremely  confusing.  In  the  nature  and  arrange- 
ment of  its  irregular  astigmatism,  every  eye  is  peculiar. 
The  number  of  varieties  of  play  of  light  and  shade  that  are 
obtainable  as  the  point  of  reversal  is  reached,  is  equal  to 
the  number  of  eyes  examined.  Even  the  two  eyes  of  the 
same  individual  differ. 

The  only  practical  way  to  deal  with  irregular  astig- 
matism by  skiascopy  is  to  understand  thoroughly  the  gen- 
eral optical  principles  of  the  test,  and  apply  them,  so  far  as 
is  needful,  in  the  individual  case.  Certain  peculiar  forms 
of  variations  of  the  refraction  of  the  eye  in  different  parts 
of  the  pupil  are,  however,  of  sufficient  constancy,  regular- 
ity and  practical  importance,  to  warrant  their  separate 
classification  and  study.  The  most  important  of  these  is 
the  regular,  or  symmetrical,  aberration  of  the  eye. 

Symmetrical  Aberration.' — This  is  an  error  of  the 
refraction  of  the  eye  which  causes  the  rays  of  an  incident 
pencil  falling  on  the  same  meridian  of  the  cornea,  but  at 
different  distances  from  the  axial  ray,  to  meet  at  different 
distances  behind  the  cornea ;  while  rays  piercing  different 
meridians  of  the  cornea,  at  the  same  distance  from  the  axial 
ray,  intersect  it  at  the  same  point.  It  is  a  defect  similar  to 
the  aberration  of  convex  and  concave  spherical  lenses.  It 
is  readily  recognizable  in  almost  all  eyes  by  skiascopy.  In 
the  majority  of  cases,  it  is  in  the  same  direction  as  ordinary 
spherical  aberration  ;  that  is,  the  margin  of  the  pupil  has  a 
stronger  lens  action  than  the  centre.  The  rays  entering 
through  the  margin  are  brought  to  a  focus  first  ;  the  rays 

^  For  an  account  of  this  error  of  refraction  see  paper  by  the  author,  Trans. 
Amer.  Ophthalmological  Society,  1888,  p.  141. 


SYMMETRICAL  ABERRATION.  59 

entering    nearer  the  centre   being   focused    farther   back. 
This  is  called  'positive  aberration. 

In  a  certain  proportion  of  cases,  however,  the  defect  is 
in  the  opposite  direction ;  the  rays  passing  near  the  centre 
of  the  pupil  being  brought  first  to  a  focus,  and  those  pass- 
ing through  the  periphery  being  focused  farther  back.  The 
centre  of  the  pupil  has  the  stronger,  and  the  periphery  of 
the  pupil  the  weaker,  lens  action.  This  is  7iegative  aberra- 
tion. 

The  Visual  Zone.— The  variation  of  refraction,  how- 
ever, does  not  usualh'  proceed  regularly  from  the  centre  of 
the  pupil  to  the  margin.     But,  as  with  spherical  lenses, 
and  to  a  greater  degree,  the  central  refraction  is  compara- 
tively uniform  over  a  considerable  area ;  and,  towards  the 
margin  the  change  of  refraction  becomes  progressiveh^  more 
marked.     This  area  in  the  centre  of  the  pupil  of  compara- 
tively uniform  refraction  is  the  usual  visual  zone.     It  is  the 
portion  of  the  pupil  that  is  of  practical  importance  for  pur- 
poses of  distinct  vision.     Its  size  varies  considerably.    Some- 
times it  includes  almost  the  whole  of  the  dilated  pupil,  in 
other  eyes  an  extremely  small  area  near  the  centre  of  the 
pupil  will  be  regular,  and  the  remainder  of  the  pupil  use- 
less  for  accurate  vision.     If   a   high  degree  of  irregular 
astigmatism  be  present,  the  visual  zone,  instead  of  behig  a 
central  area  of  considerable  size,  will  often  be  some  particu- 
lar portion  of  the  undilated  pupil,  which  happens  to  have 
the  most  regular  curvature. 

In  dny  case,  for  the  correction  of  ametropia,  it  is  the 
behavior  of  the  light  and  shade  in  the  visual  area  which 
has  to  be  studied.  Its  behavior  elsewhere  may  be  disre- 
garded. It  is  often  much  easier  to  watch  the  movement  of 
light  and  shade  in  some  other  portion  of  the  pupil— some 
part  of  the  extra-visual  zone.     And,  if  the  obser\-er  does 


60  ABERRATION  AND  IRREGULAR  ASTIGMATISM. 

not  understand  their  relative  importance,  he  will  be  apt  to 
fix  his  attention  on  this  latter  and  be  led  away  from  the 
true  refraction  of  the  eye  he  is  examining.  This  is  the 
more  likely  to  happen,  because  in  that  part  of  the  pupil, 
which  has  its  point  of  reversal  at,  or  near,  the  observer's 
eye,  the  direction  of  the  movement  of  light  and  shade  is 
difficult  to  see,  while  in  other  portions,  the  movement  is 
more  striking. 

The  Appearances  of  Positive  Aberration. — The  ap- 
pearances presented  by  an  ordinary  case  with  positive  aberra- 
tion may  be  considered  in  the  order  in  which  they  will  be 
developed  with  the  plane  mirror,  the  observer  starting  to 
examine  the  eye  from  within  the  point  of  reversal  for  the 
most  myopic  part  of  the  pupil,  and  gradually  withdrawing 
his  eve  until  it  is  beyond  the  point  of  reversal  for  the  least 
myopic  part  of  the  pupil.  From  the  first  position,  the  light 
area  in  the  pupil  is  seen  to  move  with  the  light  on  the  face 
entirely  across  the  pupil  ;  its  motions  in  the  edges  of  the 
pupil  being  more  rapid  and  indefinite  than  in  the  centre. 
If,  now,  the  observer's  eye  is  withdrawn  to  the  point  of  re- 
versal for  the  margin  of  the  pupil,  there  appear  in  the  mar- 
gin points  in  which  no  movement  of  the  light  can  be  seen. 
Some  of  these  may  be  points  of  stationary  light,  and  others, 
points  of  stationary  shadow. 

As  the  obser\'er's  eye  is  still  farther  withdrawn,  the 
points  of  stationary  light  run  together  and  form  a  complete 
ring  of  light  in  the  peripher}-  of  the  pupil,  shown  in  figure 
13,  which  is  presently  seen  to  have  an  inverted  motion,  to 
move  against  the  light  on  the  face.  Within  this  is  a  ring 
of  comparative  shadow  where  the  movement  is  swift  and 
difficult  or  impossible  to  recognize  ;  and  still  within  this 
lies  an  area  of  light,  similar  to  that  first  seen,  but  now  con- 
siderably reduced  in  size,  which  still  moves  with  the  light 
on  the  face. 


POSITIVE  ABERRATION. 


61 


As  the  observer  draws  still  farther  back,  this  area  of 
light  at  the  centre  of  the  pupil,  as  shown  in  figure  14, 
grows  smaller,  and  its  movement  more  difficult  to  certainly 
distinguish.  The  ring  of  comparative  shadow  around  it 
encroaches  upon  it,  and  the  ring  of  light  in  the  margin  of 
the  pupil  in  turn  encroaches  upon  the  shadow,  and  becomes 
brighter  and  its  movement  more  readily  noticeable,  fig.  14. 


Fig.  I- 


FlG.  14. 


Withdrawing  still  farther,  the  point  of  reversal  for  the 
•centre  of  the  pupil  is  reached.  The  central  area  of  light 
hecomes  faint  and  its  movement  ceases  to  be  noticeable, 
the  ring  of  feeble  illumination  surrounding  it  having  swal- 
lowed it  up.  But,  around  this  feeble  light  area,  the  ring 
of  inverted  movement  has  now  grown  broad  and  distinct. 
And,  as  the  observer  withdraws  still  farther,  this  ring  of 
inverted  movement  closes  in  until  it  occupies  the  whole  of 
the  central  area,  and  the  observer  sees  an  area  of  light 
moving  across  the  whole  pupil,  having  an  inverted  move- 
ment, that  is  against  the  mirror  or  light  on  the  face. 

The  movements  of  these  erect  and  inverted  light  areas 
in  the  pupil  are  illustrated  by  figures  15  and  16.  Figure 
15  shows  the  plane  mirror  turned  to  the  left,  or  the  concave 
mirror  turned  to  the  right,  the  central  erect  area  being  dis- 
placed toward  the  left,  and  the  peripheral  inverted  area  to- 
ward the  right  of  the  space  it  occupies.     Figure  16  repre- 


62  ABERRATION  AND  IRREGULAR  ASTIGxMATISM. 

sents  the  light  areas  displaced  in  the  opposite  directions  by 
an  opposite  inclination  of  the  mirror. 


Fig.  15.  Fig.  16. 

With  the  concave  mirror,  a  similar  series  of  changes 
may  be  brought  about  by  placing  before  the  eye  successive 
strengths  of  the  lenses,  beginning  with  the  weakest  convex 
or  strongest  concave.  The  first  should  allow  the  points  of 
reversal  for  all  parts  of  the  pupil  to  be  back  of  the  observer, 
and  the  successive  changes  bring  these  points  closer  and 
closer  to  the  e)'e  until  all  are  in  front  of  the  observer.  The 
movement  is  at  first  against  the  light  on  the  face.  Then 
appears  the  ring  of  illumination  and  swift  movement  in  the 
margin  of  the  pupil  with  the  light  on  the  face.  The  cen- 
tral area  of  light  is  then  encroached  upon  by  the  ring  of 
faint  illumination,  and  this  in  turn  by  a  ring  of  more  bril- 
liant illumination  in  the  margin  moving  with  the  light  on 
the  face,  which  latter  finally  occupies  the  whole  area  of  the 
pupil. 

If  the  point  of  reversal  be  approached  from  the  oppo- 
site direction,  that  is,  starting  with  the  observer's  eye  beyond 
it,  we  have,  with  the  plane  mirror,  at  first,  inverted  move- 
ment across  the  whole  pupil.  Then,  as  the  point  of  re- 
versal for  the  centre  of  the  pupil  is  approached,  the  light 
in  the  central  zone  becomes  feeble  and  its  movement  indefi- 
nite.    When  the  point  of  reversal  for  that  part  is  passed^ 


POSITIVE    ABERRATION.  63 

there  appears,  in  this  central  zone,  an  erect  movement  of 
light  and  shade,  at  first  rapid  and  hard  to  see,  but  growing 
slower,  gaining  in  distinctness,  and  occupying  a  larger  and 
larger  part  of  the  pupil  as  the  patient's  eye  is  approached, 
until,  finally,  it  occupies  the  whole  area. 

With  the  concave  mirror,  starting  with  the  strongest 
convex  or  weakest  concave  lens,  the  movement  is  first  with 
the  mirror  throughout  the  pupil,  then,  as  the  lens  is 
changed,  it  becomes  indefinite  at  the  centre  ;  presently  it  is 
against  that  of  the  mirror  at  the  centre,  while  still  with  it 
at  the  margin  ;  and,  with  still  weaker  convex  lenses,  or 
stronger  concaves,  it  becomes  against  throughout  the  whole 
pupillary  area. 

Appearances  of  Negative  Aberration — With  nega- 
tive aberration,  the  series  of  changes  is  apt  to  be  less  regu- 
lar and  complete,  and  the  picture  presented  by  the  pupil  is 
less  characteristic.  But  the  succession  of  appearances  is 
the  reverse  of  what  has  been  described  for  positive  aberra- 
tion. 

With  a  plane  mirror  starting  closer  to  the  patient's  eye 
than  the  point  of  reversal  for  the  most  myopic  part  of  the 
pupil,  the  movement  is  with  that  of  the  light  on  the  face 
throughout  the  whole  pupil.  As  the  observer's  e}-e  is  with- 
drawn to  a  greater  distance,  this  movement  becomes  indefi- 
nite, and  the  light  feeble  near  the  centre  of  the  pupil. 
Presently,  the  movement  at  the  centre  of  the  pupil  is  lost, 
while  still  quite  distinctly  with  that  of  the  light  on  the  face 
in  the  irregular  ring-shaped  area  of  the  periphery.  With- 
drawing still  farther  from  the  eye,  the  inverted  movement 
at  the  centre  of  the  pupil  becomes  distinctly  visible,  and 
the  direct  movement  near  the  margin  becomes  more  and 
more  encroached  upon  and  less  and  less  distinct,  until 
finally  all  erect  movement  is  lost  and  we  have  only  the  in- 


64  ABERRATION  AND  IRREGULAR  ASTIGMATISM. 

verted  movement,  which  extends  across  the  whole  pupil. 
Before  the  erect  movement  entirely  disappears,  it  is  apt  to 
break  up  into  small  areas  detached  from  one  another  by 
spaces  of  comparative  shadow,  but  still  presenting  some 
remnant  of  the  erect  movement. 

With  the  concave  mirror,  starting  with  a  convex  lens  so 
•weak,  or  a  concave  lens  so  strong,  that  the  point  of  reversal 
is  back  of  the  observer,  we  have  the  direct  movement 
.against  the  light  on  the  face  throughout  the  pupil.  The 
strengthening  of  the  convex  lenses  or  the  weakening  of  the 
•concaves,  so  as  to  bring  the  point  of  reversal  closer  to  the 
•obser\'er's  eye,  causes  :  first,  the  fading  of  the  light  and  the 
indefiniteness  of  its  movement  in  the  centre  of  the  pupil, 
then  the  inverted  movement,  or  with  the  light  on  the  face, 
at  the  centre,  and  the  area  of  this  movement  extending 
until  it  includes  the  whole  of  the  pupil. 

Approaching  the  point  of  reversal  from  be}'ond  it,  we 
have,  with  the  plane  mirror,  inverted  movement  through- 
out the  whole  pupil,  giving  place  to  indistinctness  first  at 
the  margin.  Then  the  indirect  movement  confined  to  the 
central  area  of  the  pupil  and  direct  movement  appearing  at 
certain  parts  of  the  marginal  area.  This  direct  movement 
becomes  more  distinct  and  its  area  increases  as  the  patient's 
eye  is  approached,  until,  at  the  point  of  reversal  for  the  cen- 
tre of  the  pupil,  all  inverted  movement  is  lost,  and  the  erect 
movement  is  seen  in  all  parts  of  the  pupillary  area. 

With  the  concave  mirror,  starting  with  the  point  of 
reversal  between  the  observer  and  patient,  and  removing  it 
successively  farther  from  the  patient,  by  the  use  of  weaker 
convex  or  stronger  concave  lenses,  we  have  first  the  move- 
ment with  the  light  on  the  face  throughout  the  pupil ; 
then  indefiniteness  at  the  pupillary  margin,  changing,  in 
turn,  to  movement  against  the  light  on  the  face.     The  area 


NEGATIVE    ABERRATION.  65 

of  this  peripheral  movement  then  encroaches  npon  the 
central  area  until  that  is  obliterated,  and  the  movement 
against  the  light  on  the  face  occupies  the  whole  width  of 
the  pupil. 

While  the  order  of  their  development  remains  the 
same,  the  exact  character  of  the  appearances  presented  var- 
ies considerably  with  the  degree  of  aberration.  Generally, 
in  the  higher  degrees,  the  areas  of  light  occupy  the  greater 
part  of  the  pupil  and  the  area  of  feeble  illumination  separ- 
ating them  is  comparativeh^  narrow.  While  in  very  low 
degrees  of  aberration,  the  area  of  feeble  illumination  is 
broad,  and  it  may  be  difficult  to  recognize  more  than  one 
of  the  light  areas  at  one  time.  That  is,  when  the  area  of 
erect  movement  is  visible,  the  remainder  of  the  pupil  is 
occupied  by  the  area  of  feeble  illumination  ;  and  when  the 
area  of  inverted  movement  is  developed,  the  area  of  feeble 
illumination  so  encroaches  upon  the  area  of  direct  move- 
ment that  it  can  no  longer  be  identified. 

In  some  eyes,  the  variation  of  refraction  from  point  to 
point  which  constitutes  symmetrical  aberration,  is  almost 
or  entirely  confined  to  the  periphery  of  the  pupil.  In  these, 
the  appearances  characteristic  of  aberration  are  hard  to  de- 
velop. 

Appearance  of  Conical  Cornea. — In  other  eyes,  an 
■opposite  condition  is  present.  The  variations  of  refraction, 
instead  of  being  confined  to  the  periphery  of  the  pupil, 
encroach  upon  the  normal  visual  zone,  confining  it  to  a 
very  narrow  area.  In  these  eyes,  the  skiascopic  appear- 
ances of  aberration  are  striking  and  characteristic,  and  one 
of  them  is  that  which  has  been  regarded  as  peculiar  to  con- 
ical cornea. 

The  error  of  refraction  produced  by  conical  cornea  is 
a  high  degree  of  negative  aberration.     At  the  apex  of  the 


66  ABERRATION  AND  IRREGULAR  ASTIGMATISM. 

cone,  the  curve  is  sharp,  causing,  usually,  very  high  myopia 
in  the  corresponding  part  of  the  pupil.  The  sides  of  the 
cone,  on  the  other  hand,  are  comparatively  flat,  causing 
diminished  myopia  as  the  region  of  the  apex  is  departed 
from  and  often  running  into  h}'peropia  near  the  edge  of  the 
pupil. 

If  the  observer's  eye  be  placed  somewhere  near  the 
point  of  reversal  for  the  periphery  of  the  pupil,  the  move- 
ment of  light  in  that  portion  of  the  pupil  will  be  rapid,  but 
the  movement  in  the  portion  of  the  pupil  corresponding  to 
the  apex  of  the  cone  will  be  slow.  On  account  of  the  high 
myopia,  the  point  of  reversal  for  this  part  of  the  pupil  is 
very  close  to  the  eye,  and,  generally,  many  dioptres  removed 
from  the  observer's  eye.  The  movement  of  light  in  the 
pupil,  then,  is  slow  near  the  centre  and  rapid  towards  the 
periphery,  causing  the  area  of  light  to  appear  to  wheel 
around  a  fixed  point  corresponding  to  the  apex  of  the  cone. 
The  light  area  is  first  seen  on  one  side  of  the  pupil,  then  on 
the  other,  but  always  rests  upon  the  central  fixed  point. 

In  certain  positions  of  the  light,  the  form  of  this  area 
will  be  somewhat  triangular,  its  base  resting  on  the  margin 
of  the  pupil  and  its  apex  at  the  apex  of  the  corneal  cone. 
Sometimes  the  triangle  covers  almost  half  of  the  pupil,  in 
other  conditions  of  light  it  is  considerably  narrower,  but 
the  constant  and  characteristic  phenomena  is  the  wheeling 
of  the  light  area  about  the  fixed  point  at  the  apex. 

This  is  shown  in  figures  17  and  18,  which  represent 
the  appearance  of  the  pupil  with  the  mirror  inclined  in 
opposite  directions. 

It  was  for  the  detection  of  these  appearances,  to  which 
attention  was  called  by  Bowman,  in  1857,  ^^^^  ^^^  test  was 
first  employed.  Bowman  mentions  that  he  was  able  by 
means  of  it  to  detect  low  degrees  of  conical  cornea,  which 


APPEARANCE  OF  CONICAL  CORNEA.  67 

would  not  be  detected  in  any  other  way.  It  is  certain  that 
among  those  cases  that  have  been  classed  as  low  degrees  of 
conical  cornea,  on  account  of  their  presenting  such  appear- 


FiG.  17-  Fig.  i8. 

ances,  a  considerable  proportion  were  not  of  conical  cornea 
at  all,  but  were  cases  of  high  aberration  from  other  forms 
of  defect  in  the  dioptic  surfaces. 

The  appearances  in  question  occur  in  all  cases  of  high 
aberration.  Where  the  aberration  invades  the  central  por- 
tion of  the  pupil,  and  is  not  confined  to  the  periphery,  the 
phenomena  are  quite  as  striking  and  characteristic,  and  of 
very  much  more  frequent  occurrence  in  cases  of  high  posi- 
tive aberration  than  in  cases  of  true  conical  cornea.  The 
conditions  for  their  recognition  are  that  the  observer's  eye 
shall  be  comparatively  near  the  point  of  reversal  for  the 
margin  of  the  pupil,  and  comparatively  far  removed  (esti- 
mating by  dioptres)  from  the  point  of  reversal  for  the  cen- 
tre of  the  pupil.  By  careful  management  of  the  light  and 
relative  position  of  observer  and  patient,  such  appearances 
can  be  demonstrated  in  the  majority  of  eyes. 

Like  the  band-like  appearances  of  the  light  in  astig- 
matism, those  of  conical  cornea  reveal  the  presence  of  the 
condition  and  the  location  of  the  apex  of  the  cone,  but  be- 
yond this,  they  are  of  little  value.  The  measurement  of 
the  difference  of  refraction   between  the  margin  and  the 


68  ABERRATION  AND  IRREGULAR  ASTIGMATISM. 

centre  of  the  pupil,  or  the  measurement  of  the  refraction  in 
the  portion  of  the  pupil  best  suited  to  purposes  of  vision, 
must  be  accomplished  by  the  same  application  of  skiascopy 
as  serves  to  measure  the  amount  of  hyperopia  or  myopia  in 
an  eye  free  from  astigmatism  and  aberration. 

The  series  of  movements  presented  in  positive  aberra- 
tion can  be  well  studied  in  one  of  the  numerous  forms  of 
artificial  eyes,  in  which  spherical  lenses  are  used  to  repre- 
sent the  dioptric  surfaces,  and  it  is  well  by  such  study  to 
become  thoroughly  familiar  with  them.  They  may,  of 
course,  be  studied  in  living  eyes  presenting  positive  aberra- 
tion ;  but,  in  many  of  these,  the  appearances  are  not  so 
typical  and  regular  in  order  of  sequence,  as  with  the  ordi- 
nary strong  spherical  lenses.  The  appearances  presented 
by  negative  aberration  can  only  be  studied  in  eyes  in  which 
this  condition  of  the  refraction  is  present,  but  their  recog- 
nition and  observation  will  be  comparatively  easy  to  one 
who  has  mastered  the  corresponding  appearances  of  positive 
aberration,  and  who  understands  the  optical  conditions  on 
which  these  appearances  depend. 

Scissors-like  Movement. — A  special  form  of  irregular 
astigmatism  exists  of  sufhciently  frequent  occurrence  and 
striking  character  to  merit  special  description.  It  is  also 
of  some  practical  importance.  In  it,  one  portion  of  the 
pupil,  as  an  upper  or  lower  half,  is  more  myopic  in  a  cer- 
tain meridian  than  is  the  other  part  of  the  pupil.  This 
causes  an  inverted  movement  of  light  in  the  one  portion  of 
the  pupil,  while  there  is  an  erect  movement  in  the  other. 
These  two  areas  are  distinct  and  separated  by  an  intermedi- 
ate zone  of  feeble  illumination.  As  the  light  is  made  to 
move  back  and  forth  in  this  meridian,  the  two  areas  of 
light  in  the  pupil  are  seen  alternately  to  approach  and  sep- 
arate, narrowing  or  widening  the  intermediate  zone.     As 


SCISSORS- LIKE  MOVEMENT. 


69 


the  areas,  under  these  circumstances,  are  generally  band- 
like,  or  have  comparatively  straight  margins,  the  effect 
is  similar  to  that  of  the  opening  and  closing  of  a  pair  of 
scissors.  These  appearances  are  represented  in  figure  19, 
which  shows  the  mirror  so  turned  as  to  separate  the  two 
areas  ;  and  figure  20,  which  represents  them  brought  to- 
gether by  an  opposite  inclination  of  the  mirror.  Suppos- 
ing the  upper  part  of  the  pupil  to  be  more  myopic,  figure 
19  corresponds  to  the  plane  mirror  facing  down  or  the  con- 
cave mirror  facing  up ;  and  figure  20  shows  the  plane  mir- 
ror facing  up  or  the  concave  mirror  facing  down. 


Fig.  19. 


Fig.  20. 


The  relative  size  of  the  two  areas  will  depend  on  the 
distance  of  the  observer  from  the  eye  or  upon  the  strength 
of  the  lens  employed.  As  the  observer  withdraws  to  a 
greater  distance,  or  the  convex  lens  is  made  stronger,  or  the 
concave  lens  is  made  weaker,  the  area  of  inverted  move- 
ment encroaches  upon  the  zone  of  feeble  illumination  sep- 
arating the  areas  of  light  and  the  area  of  erect  movement 
diminishes.  As  the  observer  comes  closer  to  the  eye,  or 
the  convex  lens  is  made  weaker  or  the  concave  lens  stronger, 
the  area  of  inverted  movement  diminishes.  Always  the 
observer's  eye  is  at  or  near  the  point  of  reversal  for  the  por- 
tion of  the  pupil  occupied  by  the  intermediate  zone  of 
feeble  illumination  ;  and,  in  making  the  determination  of 


70  ABERRATION  AND  IRREGULAR  ASTIGMATISM. 

the  refraction  for  practical  purposes,  care  must  be  taken  to 
see  that  this  zone  occupies  a  portion  of  the  pupil  that  is 
available  when  the  pupil  is  contracted,  as  under  ordinary 
conditions  of  illumination  and  near  work. 

The  scissors-like  movement  may  be  produced  in  an 
artificial  eye  by  placing  the  lens  which  represents  the  diop- 
tric surfaces,  so  that  the  light  passes  through  it  obliquely. 
It  may  also  be  developed  in  most  eyes  by  applying  skias- 
copy^ from  some  direction  at  a  considerable  angle  to  the 
optic  axis.  Its  presence  in  the  eye  indicates  obliquity  of 
one  or  more  of  the  dioptric  siirfaces.  Probably  it  is  often 
due  to  some  obliquity  in  the  position  of  the  crystalline  lens. 
Perhaps,  because  of  such  obliquity,  this  appearance  of  light 
and  shadow  in  the  pupil  is  apt  to  co-exist  with  a  consider- 
able degree  of  regular  astigmatism,  which,  on  account  of  it, 
becomes  more  diihcult  to  recognize  and  measure  than  it 
would  otherwise  be.  Eyes  presenting  it,  therefore,  demand 
special  care  and  attention  on  the  part  of  the  observer,  to 
develop  their  best  vision  possible  with  correcting  lenses. 


CHAPTER  VI. 

PRACTICAL   APPLICATION    WITH    THE    PLANE    MIRROR. 

Position  and  Arrangement  of  Light. — The  room  being 
thoroughly  darkened,  the  patient  and  surgeon  take  posi- 
tions facing  each  other  at  a  distance  of  about  one  metre 
with  the  original  source  of  light  close  to  the  surgeon  on 
the  side  of  the  eye  he  desires  to  use,  that  is  on  the  right  if 
he  intends  using  his  right  eye  for  the  test.  He  can  really 
use  but  one  eye  at  a  time,  although  he  will  find  it  much 
more  pleasant  to  work  with  both  eyes  open  if  he  once  learns 
to  do  so.  The  source  of  light  should  be  freely  movable 
over  a  space  of  about  one  metre,  a  movement  obtainable 
with  a  double  jointed  bracket  of  over  one-half  metre  total 
length.  The  light  is  covered  from  the  patient's  face  and 
also  from  the  surgeon's  except  at  the  aperture  of  five  [or 
ten]  millimetres  opposite  the  brightest  part  of  the  flame. 

The  mirror  is  held  so  that  with  the  eye  he  is  using  the 
surgeon  can  watch  through  the  sight  hole  the  movement 
of  light  on  the  patient's  face  and  turned  until  the  area  of 
light  that  it  reflects  falls  upon  the  eye  to  be  tested.  If  dif- 
ficulty is  experienced  in  properly  directing  the  light,  the 
surgeon  may  hold  his  hand  a  few  inches  in  front  of  the 
mirror  and  upon  it  find  the  light  area  and  get  it  properly 
directed  towards  the  patient's  eye.  If  the  mirror  be  large 
it  is  necessary  that  the  central  portion  of  the  light  area  be 
made  to  fall  upon  the  patient's  eye,  the  centre  being  marked 

(71) 


72  APPLICATION  WITH  THE  PLANE  MIRROR. 

by  a  spot  of  feeble  illumination,  corresponding  to  the  sight 
hole  of  the  mirror. 

With  the  light  properly  directed,  the  pupil  will  be 
seen  to  be  occupied  by  a  red  glare,  the  light  area  with 
which  skiascopy  is  especially  concerned.  In  first  attempt-^ 
ing  the  test,  care  must  be  taken  to  discriminate  clearly  be- 
tween this  general  red  glare  and  the  reflection  from  the 
cornea  or  from  the  surfaces  of  any  lens  that  may  be  placed 
before  the  eye.  These  reflections  have  the  same  color  as 
the  ligfht  used  for  the  test.  The  one  from  the  cornea  is 
small  and  brilliant,  a  mere  point  of  light  if  the  room  be 
thoroughly  darkened  and  the  original  source  of  light  prop- 
erly shaded.  The  reflections  from  the  lenses  employed  are 
larger  and  more  confusing.  They  may  be  avoided  by  tilt- 
ing the  lens  slightly,  in  which  case,  they  pass  off  to  the  peri- 
phery, leaving  the  centre  of  the  lens  free  from  reflection. 

Hyperopia. — If  the  mirror  be  rotated  about  a  vertical 
axis,  that  is  if  it  be  made  to  turn  more  to  the  right  or  left, 
the  area  of  light  in  the  pupil  will  be  seen  to  move  with  the 
light  on  the  face  to  the  right  or  left  as  the  inclination  of 
the  mirror  changes.  If  the  rate  of  movement  be  slow,  the 
h)peropia  is  of  high  degree,  if  more  rapid,  it  is  lower, 

A  convex  lens  is  now  to  be  placed  before  the  eye  and 
this  rate  of  movement  of  light  in  the  pupil  is  the  guide  to 
the  probable  strength  of  lens  required.  If  the  observer  has 
not  sufficient  practice  with  skiascopy  to  judge  in  this  way 
about  the  strength  of  the  lens  required,  he  will  save  time 
by  placing  before  the  eye  rather  a  strong  lens,  one  of  say 
5  D.  With  this  the  light  is  again  thrown  upon  the  eye, 
and  if  the  lens  be  not  sufiicient  to  correct  the  hyperopia 
present,  the  movement  of  light  in  the  pupil  will  still  be 
found  with  that  of  the  light  on  the  face.  In  this  case  a 
still  stronger  lens  must  be  used.     This  strengthening  of 


HYPEROPIA.  73 

the  convex  lens  before  the  eye  is  continued  until  one  is 
found  which  causes  the  reversal  of  the  apparent  movement 
of  light  in  the  pupil — until  the  light  in  the  pupil  moves 
against  the  light  on  the  face. 

Then  the  surgeon  is  to  approach  the  patient,  mean- 
while rotating  the  mirror  and  watching  for  the  nearest  point 
at  which  he  still  sees  the  inverted  movement  in  the  visual 
zone.  Near  this  distance,  the  illumination  of  the  pupil  be- 
comes quite  feeble,  and  the  movement  being  rapid  requires 
the  closest  watching.  Approaching  still  nearer  to  the 
patient  the  light  in  the  visual  zone  is  seen  to  move  with  the 
light  on  the  face,  and  the  greatest  distance  at  which  this 
can  be  distinguished  is  to  be  noted.  Between  these  two, 
the  least  distance  of  inverted  movement,  and  the  greatest 
distance  of  direct  movement  lies  the  point  of  reversal. 
But,  for  reasons  given  page  40,  it  is  better  to  take  the  lat- 
ter, the  greatest  distance  at  which  direct  movement  can  be 
perceived  as  the  point  sought. 

The  distance  from  the  surgeon's  to  the  patient's  eye- 
is  then  measured.  It  is  the  focal  distance  of  the  amount  of 
myopia  produced  by  the  convex  lens  employed.  That 
amount  is  to  be  subtracted  from  the  total  strength  of  the 
lens  to  ascertain  the  portion  of  its  strength  which  has. 
been  necessary  to  correct  the  existing  hyperopia. 

Having  made  such  a  determination  of  the  refractiom 
and  having  repeated  the  various  observ^ations  until  no- 
doubt  is  left  as  to  their  correctness,  the  lens  before  the  eye 
is  to  be  changed  for  one  sufficiently  weaker  to  carry  the 
point  of  reversal  to  as  great  distance  as  the  size  of  the 
visual  zone  will  allow  the  accurate  determination  of  the 
movements  of  light  and  shade  within  it.  At  this  distance 
the  final  estimate  of  the  ametropia  is  to  be  completed. 

For  example,  suppose  the  eye  under  examination  to 
6 


74  APPLICATION  WITH  THE  PLANE  MIRROR. 

have  hyperopia  of  3  D.  When  the  5  D.  lens  is  placed  be- 
fore it,  the  point  of  reversal  will  be  brought  to  one-half 
metre.  As  the  surgeon's  eye  is  made  to  approach  that  of 
the  patient,  the  inverted  movement  in  the  visual  zone  will 
cease  when  they  are  about  60  centimetres  apart.  Going 
still  closer,  the  erect  movement  will  be  distinguished  at 
about  40  centimeters.  These  observations  are  to  be  repeated 
until  the  surgeon  makes  sure  that  the  point  of  reversal  lies 
somewhere  between  40  and  60  centimetres.  The  5  D.  lens 
is  then  replaced  by  the  4  D.  lens.  Repeating  the  test,  the 
inverted  movement  is  seen  at  one  and  one-quarter  metres 
and  the  direct  movement  as  far  away  as  one  metre,  thus 
locating  the  point  of  reversal  at  about  i  metre  from  the 
eye,  and  determining  the  myopia  caused  by  a  4  D.  convex 
lens  to  be  i  D.  and  the  refraction  of  the  eye  to  be  4  D.- 
I  D.^3  D.  of  hyperopia  with  less  than  0.25  D.  of  possible 
error  either  way. 

Myopia. — In  myopia  the  first  rotation  of  the  mirror 
will  usually  cause  a  movement  of  light  in  the  pupil  against 
that  of  the  light  on  the  face.  The  surgeon  then  approaches 
the  patient,  continuing  the  movements  of  the  mirror  and 
watching  the  apparent  movement  of  light  in  the  pupil, 
until  this  apparent  movement  becomes  rapid  and  indefinite 
and  presently  is  entirely  lost.  Approaching  still  closer  to 
the  patient's  eye,  the  movement  of  the  light  area  in  the 
pupil  again  becomes  distinct,  but  is  now  with  the  move- 
ment of  the  light  on  the  face.  Drawing  back  again,  the 
surgeon  notes  the  greatest  distance  at  which  this  erect 
movement  can  be  observed,  and  the  shortest  distance  at 
which  the  inverted  movement  is  distinguishable,  and  takes 
a  point  midway  between  these  to  be  the  point  of  reversal. 

The  distance  of  this  point  of  reversal  from  the  patient's 
eye  is  the  focal  distance  of  the  lens  that  will  be  required  to 


MYOPIA.  75 

correct  the  myopia.  To  complete  the  test,  however,  a  lens 
about  I  D.  weaker  than  this  is  placed  before  the  eye  to 
bring  the  point  of  reversal  to  the  distance  of  a  metre  and 
the  test  is  repeated,  the  surgeon  noting  carefully  the  great- 
est distance  at  which  the  erect  movement  is  visible,  and 
the  shortest  distance  at  which  the  inverted  movement  is 
perceived,  always  in  the  visual  zone.  The  distance  of  the 
point  of  reversal  as  thus  determined  is  the  focal  distance  of 
the  lens  required  to  correct  the  remaining  myopia.  The 
strength  of  such  a  lens  added  to  the  strength  of  the  lens 
already  before  the  eye,  gives  the  total  amount  of  myopia 
present. 

Suppose  the  eye  to  be  6.5  D.  myopic.  With  the  first 
test  the  inverted  movement  will  be  perceived  up  to  about 
eight  inches  from  the  patient's  eye  ;  and  at  five  or  six  inches 
from  the  eye  an  erect  movement  will  begin.  From  this, 
the  surgeon  assumes  that  the  m}'opia  is  about  7  D.  [focal 
distance  6^  inches]  and  he  will,  therefore,  place  before  the 
eye,  for  the  more  accurate  test,  a  concave  6  D.  lens.  On 
trying  the  movement  of  light  in  the  pupil  through  this 
lens,  it  will  be  found  at  the  distance  of  one  metre  to  be 
with  that  of  the  light  on  the  face.  The  surgeon  then  with- 
draws still  farther  from  the  patient  until  the  direct  move- 
ment becomes  indistinguishable  and  at  two  metres  is  entirely 
lost.  Drawing  back  still  farther  from  the  patient,  he  will 
in  a  favorable  eye  be  able  to  distinguish  the  inverted  move- 
ment in  the  pupil,  and  in  this  way  fix  the  point  of  reversal 
at  a  distance  of  two  metres,  indicating  with  great  accuracy 
an  uncorrected  myopia  of  0.5  D. 

Sometimes,  however,  the  distance  of  two  metres  will 
be  found  so  great  that  it  is  difficult  or  impossible  there  to  be 
sure  of  the  movement  in  the  visual  zone.  In  such  a  case 
the  6  D.  lens  will  need  to  be  replaced  by  a  weaker  lens  as 


76  APPLICATION  WITH  THE  PLANE  MIRROR. 

a  5.5  D.,  with  which  the  erect  movement  will  be  seen  to 
almost  a  metre,  and  the  inverted  movement  again  a  few 
inches  beyond  that  point. 

If  the  myopia  be  very  low,  the  first  inspection  of  the 
pupil  without  a  lens  may  show  a  movement  of  light  in  it 
ivith  the  light  on  the  face.  In  such  a  case,  the  surgeon 
will  draw  back  as  far  as  he  can  readily  distinguish  the 
movement  of  light  in  the  visual  zone.  If  the  movement 
still  appears  to  be  ivith  that  of  the  light  on  the  face,  he  will 
place  before  the  eye  a  convex  lens,  and  with  it  determine  the 
point  of  reversal  as  for  a  case  of  hyperopia.  The  final 
result  of  testing,  however,  will  show  that  the  myopia  caused 
by  the  lens  is  greater  than  the  strength  of  the  lens,  and, 
therefore,  that  some  myopia  must  have  been  present  before 
the  lens  was  placed  in  front  of  the  eye. 

For  example,  suppose  that  before  reaching  that  dis- 
tance of  two  metres  the  erect  movement  in  the  pupil 
becomes  indistinct,  but  that  the  visual  zone,  where  the 
movement  must  be  watched,  is  so  small  that  beyond  this 
the  direction  of  movement  in  it  cannot  be  recognized  with 
certaintv.  A  0.5  D.  convex  lens  being  placed  before  the 
eye  is  found  to  cause  an  inverted  movement  up  to  125  cen- 
timetres, and  to  confine  the  erect  movement  to  within  85 
or  90  centimetres  of  the  eye.  The  point  of  reversal  then, 
is  at  one  metre.  The  amount  of  myopia  corresponding  to 
this  is  I  D.,  of  which  0.5  D.  was  the  amount  originally 
present  in  the  eye. 

Emmetropia. — On  first  inspection,  without  a  lens,  the 
surgeon  sees  an  erect  movement  in  the  pupil,  the  rapidity 
of  which  indicates  that  if  there  be  hyperopia  it  is  of  low 
degree.  Drawing  back  from  the  patient's  eye  as  far  as  pos- 
sible, however,  this  erect  movement  still  continues.  He 
places  before  the  eye  under  observation  a  convex  lens  of  i 


EMMETROPIA. 


or  2  D.,  and  viewing  the  movement  of  light  in  the  pupil 
through  this  lens,  finds  where  the  inverted  and  the  erect 
movement  come  together.  On  measuring  the  distance  of 
this  point  of  reversal  from  the  patient's  eye,  he  finds  that 
it  exactly  corresponds  with  the  focal  distance  of  the  lens  he 
has  been  using.  That  is,  the  lens  has  caused  myopia  just 
•equivalent  to  its  own  strength,  showing  that  before  they 
passed  through  the  lens,  the  rays  emerging  from  the  cornea 
were  parallel. 

Regular  Astigmatism. — Whether  it  be  known  that 
the  eye  under  examination  is  astigmatic  or  not,  the  test 
will  proceed  at  first  as  for  simple  hyperopia  or  myopia. 
Sometimes  if  the  astigmatism  be  high  and  one  meridian 
nearly  emmetropic  or  slightly  myopic,  the  first  inspection, 
without  any  lens,  will  reveal  an  unmistakable  band  of  light, 
or  that  there  is  erect  movement  in  one  meridian  and  in- 
verted movement  in  another,  or  that  the  movement  of 
light  in  the  pupil  is  more  rapid  in  some  one  meridian  than 
in  the  meridian  at  right  angles  to  it,  indicating  that  these 
meridians  have  different  points  of  reversal,  and  that  the 
surgeon  is  nearer  the  point  of  reversal  for  the  former  than 
for  the  latter. 

But,  commonly,  the  first  appearance  will  give  no  posi- 
tive indication  of  the  presence  of  astigmatism,  and  the  test 
goes  on  until  a  point  of  reversal  is  found.  Then,  on  trying 
the  movement  of  light  and  shade  in  different  meridians,  as 
should  alwa}'S  be  done  from  the  neighborhood  of  the  point 
of  reversal,  it  is  discovered  that  it  is  the  point  of  reversal 
for  only  one  meridian ;  and  that  for  the  meridian  at  right 
angles  to  that  one,  there  is  a  distinct  movement  of  the  light 
either  erect  or  inverted. 

If  the  movement  still  noticeable  from  the  point  of 
reversal  first  discovered  be  an  inverted  movement — against 


78  APPLICATION  WITH  THE  PLANE  MIRROR. 

the  light  on  the  face — the  surgeon  should  bring  his  eye 
still  closer  to  the  patient  until  this  inverted  movement 
ceases.  He  will  then  be  near  the  point  of  reversal  for  the 
meridian  in  which  the  inverted  movement  was  before 
noticed,  and  will  be  able  to  see  in  the  other  meridian  an 
erect  movement. 

Such  a  lens  is  now  to  be  chosen  and  placed  before  the 
eye  as  will  bring  this  point  of  reversal  for  the  more  myopic 
meridian — the  point  of  reversal  from  which  an  erect  move- 
ment is  seen  in  the  other  meridian — to  a  convenient  distance 
from  the  eye.  The  surgeon's  eye  is  placed  as  nearly  as  pos- 
sible at  this  point  of  reversal.  Then  the  original  source 
of  light  [which  has  up  to  this  stage  of  the  test  accompanied 
the  mirror  in  its  movements  to  or  from  the  patient's  eye] 
is  pushed  away  from  the  mirror,  and  while  it  is  pushed 
away,  the  mirror  is  rotated  and  the  light  area  in  the  pupil 
watched.  This  light  area  will  be  seen  to  assume  the  band- 
like appearance  characteristic  of  astigmatism. 

At  a  certain  distance  this  band-like  appearance  will  be 
most  distinct.  With  the  source  of  light  nearer  the  mirror 
or  farther  from  the  mirror,  it  will  be  less  characteristic. 
The  distance  of  the  light  from  the  mirror  at  which  the 
band  becomes  most  distinct  is  the  distance  between  the  two 
points  of  reversal.  The  surgeon's  eye  (with  the  mirror)  is 
now  at  the  point  of  reversal  for  the  more  myopic  meridian, 
and  the  immediate  source  of  light  is  at  the  point  of  rever- 
sal for  the  less  myopic  meridian. 

With  the  light  in  this  position,  the  direction  of  the 
band  is  to  be  carefully  studied  and  noted  as  the  direction 
of  one  of  the  principal  meridians  of  astigmatism.  It  is  the 
direction  of  the  axis  of  the  convex  cylinder  that  will  cor- 
rect the  astigmatism.  The  other  principal  meridian  will,, 
of  course,  be  perpendicular  to  this. 


REGULAR  ASTIGMATISM.  79 

Having  now  fixed  the  direction  of  the  principal  me- 
ridians of  astigmatism,  the  surgeon  should  again  bring  the 
original  source  of  light  as  near  to  the  mirror  as  possible, 
and  proceed  to  measure  the  refraction,  first  in  the  one  prin- 
cipal meridian  and  then  in  the  other,  just  as  he  would 
measure  the  refraction  in  cases  of  hyperopia  or  myopia. 
The  difference  between  the  refractions  of  the  two  being  the 
amount  of  astigmatism  present. 

To  measure  the  refraction  in  a  certain  meridian  the 
light  is  made  to  move  on  the  face  and  on  the  retina  in  the 
direction  of  this  meridian  by  rotating  the  mirror  about  an 
axis  perpendicular  to  it.  Thus  for  the  vertical  meridian 
the  light  is  made  to  move  vertically  by  turning  the  mirror 
about  a  horizontal  axis.  For  the  horizontal  meridian  the 
light  is  made  to  move  horizontally  about  a  vertical  axis. 
Great  care  is  necessary  in  the  higher  degrees  of  astigmatism 
to  make  the  movement  conform  accurately  to  the  meridian 
to  be  tested,  since  any  oblique  movement  will  appear  (see 
page  55)  as  though  perpendicular  to  the  band. 

When  the  astigmatism  is  of  very  low  degree,  0.5  D.  or 
less,  it  becomes  correspondingly  difficult  to  distinguish  be- 
tween the  points  of  reversal  for  its  principal  meridians. 
The  band-like  appearance  of  the  light  in  the  pupil  becomes 
less  characteristic,  and  there  is  no  space  between  the  two 
points  of  reversal  where  an  erect  movement  can  be  obtained 
in  the  direction  of  one  meridian,  and  a  reverse  movement 
in  the  direction  of  the  meridian  perpendicular  to  it.  In 
these  cases,  the  astigmatism  is  to  be  recognized  by  the  fact 
that  when  near  one  point  of  reversal,  the  movement  in  one 
meridian  has  become  indistinguishable,  it  can  still  be  per- 
ceived in  the  other  principal  meridian.  And,  if  the  sur- 
geon places  his  eye  at  the  point  of  reversal  for  the  more 
myopic   meridian  and  pushes  the  source  of  light  a  little 


80  APPLICATION  WITH  THE  PLANE  MIRROR. 

away  from  the  mirror,  the  erect  movement  in  the  meridian 
of  less  myopia,  and  absence  of  movement  in  the  more 
myopic  meridian  becomes  most  distinct.  It  is  upon  the 
behavior  of  light  in  the  pnpil  under  these  conditions  that 
the  diagnosis  of  the  very  low  degrees  of  astigmatism  must 
principally  rest. 

The  final  test  in  any  case  will  be  made  with  the  points 
of  reversal  brought  together,  usually  at  a  distance  of  i 
metre  or  more.  To  do  this,  it  will  be  necessary  to  place 
"before  the  eye  such  a  cylindrical  lens  as  will  correct  the 
.astigmatism,  together  with  the  spherical  lens  which  will 
bring  the  point  of  reversal  to  the  desired  distance.  With 
these  lenses  before  the  eye,  the  test  is  again  applied.  If 
the  light  in  the  pupil  is  found  to  move  with  the  light  on 
the  face,  the  surgeon  withdraws  to  a  greater  distance  until 
that  movement  becomes  indistinct.  If  the  movement  in 
the  pupil  is  found  against  that  of  the  light  on  the  face,  the 
surgeon  approaches  the  patient  until  the  movement  be- 
comes indistinct.  The  apparent  movement  is  to  be  care- 
fully inspected  from  the  point  of  reversal  and  from  a  little 
within  and  a  little  beyond  it. 

If  it  is  found  that  the  reversal  occurs  at  the  same  dis- 
tance from  the  eye  for  all  meridians,  the  cylinder  chosen  is 
known  to  be  correct,  both  as  to  strength  and  as  to  the 
placing  of  its  axis  ;  and  the  distance  of  this  point  of  rever- 
sal from  the  eye  indicates  the  amount  of  myopia  which  the 
spherical  lens  employed  has  caused  or  has  left  uncorrected. 

If,  however,  the  movement  of  light  is  found  to  cease 
in  some  meridian,  but  to  continue  (either  direct  or  inverted) 
in  a  meridian  at  right  angles  thereto,  it  becomes  evident 
that  the  cylinder  chosen  does  not  perfectly  correct  the 
astigmatism.  If  the  astigmatism  thus  found  to  remain  has 
the  same  principal  meridians  as  those  already  fixed  upon, 


REGULAR    ASTIGMATISM.  81 

the  direction  of  the  axis  of  the  lens  is  correct,  but  its 
strength  is  not  exactly  right.  Whether  the  strength  needs 
to  be  increased  or  to  be  diminished  will  appear  from  the 
fact  that  the  more  myopic  meridian  continues  to  be  the 
more  m}'opic  ;  or  that  what  was  originally  the  less  myopic 
meridian  has  become  the  more  myopic. 

If  the  astigmatism  remaining  after  the  cylindrical  lens 
has  been  placed  before  the  eye  has  principal  meridians  that 
do  not  correspond  with  those  for  which  the  lens  is  placed, 
the  placing  of  the  lens  is  incorrect,  and  the  direction  of  its 
axis  needs  to  be  slightly  varied,  until  the  remaining  astig- 
matism disappears  or  its  direction  corresponds  with  that  of 
the  lens  before  the  eye. 

Where  the  cylindrical  lens  before  the  eye  is  of  the 
right  strength  or  is  too  weak,  its  axis  needs  to  be  turned 
slightly  toward  the  axis  of  a  similar  cylinder  which  would 
correct  the  remaining  astigmatism.  If  the  cylindrical  lens 
already  before  the  eye  is  too  strong,  its  axis  needs  to  be 
turned  slightly  toward  the  axis  of  a  cylindrical  lens  of  the 
■opposite  kind  that  would  correct  the  astigmatism. 

The  effect  of  such  combinations  of  cylindrical  lenses 
may  be  more  fully  understood  by  the  study  of  the  writer's 
paper  upon  "  The  Equivalence  of  Cylindrical  and  Sphero- 
cylindrical Lenses  "  in  the  Transactions  of  the  American  Oph- 
thalmological  Society  for  1886,  page  268,  or  "  Some  Remarks 
on  the  Refractive  Value  of  two  Cylinders  "  by  Carl  W^eiland 
Archivef,  of  OphthalmoJogy,  1893,  p.  435,  and  1894,  page  28. 

When  the  meridians  of  any  remaining  astigmatism 
hiave  thus  been  made  to  conform  to  the  direction  of  the 
cylindrical  lens  before  the  eye,  this  remaining  astigmatism 
has  to  be  corrected  by  a  change  in  the  strength  of  the  cylin- 
drical lens. 

For    example  :  suppose    an    eye  having  a  compound 


82  APPLICATION  WITH  THE  PLANE  MIRROR. 

hyperopic  astigmatism  corrected  by  +  i  sph.  3  ^~  ^  ^y^ 
axis  95°.  The  first  inspection  of  the  movement  of  light 
in  the  pupil  shows  a  movement  with  that  of  the  light  on 
the  face  in  all  meridians ;  and  the  difference  in  the  rate  of 
movement  in  the  different  meridians  will  be  so  slight  as 
probably  to  escape  notice.  A  convex  3  D.  spherical  lens 
will  cause  the  movement  in  the  pupil  to  be  against  the  light 
on  the  face  in  all  meridians  when  the  eye  is  viewed  from  a 
greater  distance  than  one  metre.  But  it  will  also  be  noticed 
that  the  light  moves  more  swiftly  from  side  to  side  than  it 
does  upward  and  downward. 

If  now  the  surgeon  brings  his  eye  closer  to  the  patient, 
when  the  distance  of  one  metre  is  reached,  the  movement 
of  the  light  from  side  to  side  becomes  indistinguishable, 
while  there  is  still  a  very  distinct  movement  against  the 
light  on  the  face  upward  and  downward.  Approaching 
still  closer,  the  movement  from  side  to  side  is  seen  to  be 
witJt  the  movement  of  the  light  on  the  face,  the  inverted 
movement  still  continuing  in  the  vertical  meridian.  The 
movement  horizontally  tvitli  the  light  on  the  face,  at  first 
very  rapid,  grows  slower  as  the  patient's  eye  is  approached, 
and  the  movement — against  the  light  on  the  face — in  the 
vertical  meridian  grows  more  rapid,  until  at  a  distance  of 
one-half  of  a  metre,  the  movement  in  the  vertical  meridian 
becomes  indistinguishable,  although  there  is  a  very  clear 
movement  of  light  ivitJt  the  light  on  the  face  from  side  to 
side. 

The  point  of  reversal  for  the  more  myopic  meridian 
(more  myopic  with  the  lens)  has  now  been  reached,  and  the 
the  surgeon  keeping  his  eye  at  this  position  pushes  the 
source  of  light  away  from  the  mirror.  As  he  does  so,  the 
area  of  light  in  the  pupil  assumes  more  and  more  the 
appearance  of  a  distinct  vertical  band,  readily  moved  from 


REGULAR    ASTIGMATISM.  83 

side  to  side,  but  without  apparent  movement  in  the  direc- 
tion of  its  length.  This  band  continues  to  become  more 
and  more  distinct  until  the  original  source  of  light  is  one- 
half  metre  from  the  mirror ;  and  the  immediate  source 
consequently  one-half  metre  back  of  the  mirror,  and  one 
metre  from  the  patient's  eye — at  the  point  of  reversal  for 
the  less  myopic  meridian.  In  this  position  careful  obser- 
vations will  show  that  the  band  of  light  in  the  pupil  is  not 
exactly  vertical,  but  has  the  direction  corresponding  to  the 
more  myopic  meridian  of  95°.  The  principal  meridians 
then  are  located  at  5°  and  at  95°. 

Having  determined  this,  the  light  is  brought  back  as 
close  to  the  mirror  as  possible,  and  the  point  of  reversal  for 
the  95°  meridian  is  determined.  To  do  this  it  may  be 
advisable  to  change  the  convex  spherical  lens  before  the 
eye,  but  whatever  lens  is  employed,  from  the  results 
obtained  with  it,  the  surgeon  deduces  the  fact  that  in  that 
meridian  the  refraction  of  the  eye  is  hyperopic  i  D.  He 
then  proceeds  to  measure  in  the  same  manner  the  refraction 
of  the  eye  in  the  other  principal  meridian,  finding  with  the 
convex  3  D.  lens  that  this  point  of  reversal  is  at  one  metre, 
and  its  refraction,  therefore,  hyperopic  2  D.  The  differ- 
ence between  these  meridians  will  be  i  D.,  the  amount  of 
astigmatism  present. 

To  make  the  final  determination  there,  there  should  be 
placed  before  the  patient's  eye  the  i  D.  convex  cylinder 
with  its  axis  at  95°  and  a  2  D.  convex  spherical  lens,  with 
which  the  point  of  reversal  for  all  meridians  will  be  found 
to  lie  one  metre  from  the  eye.  If,  in  the  placing  of  the 
cylinder,  its  axis  is  made  to  not  correspond  exactly  with 
the  meridian  of  least  hyperopia,  there  will  be  found  by 
this  test  a  remaining  astigmatism  of  low  degree.  Suppose 
through  carelessness  or  inaccuracy  in  the  earlier  observa- 


84  APPLICATION  WITH  THE  PLANE  MIRROR. 

tion,  the  axis  of  the  cylinder  should  be  placed  at  105°  in- 
stead of  95°,  the  remaining  astigmatism  then  would  be 
found  to  be  such  as  would  be  corrected  by  a  convex  cylin- 
der with  its  axis  at  about  70°.  But  on  turning  the  cylin- 
der before  the  eye  10°  in  that  direction,  that  is,  to  its  proper 
direction  at  95°,  the  remaining  astigmatism  would  disap- 
pear. If,  however,  instead  of  the  i  D.  cylindrical  lens,  a 
lens  of  1.5  D.  had  been  placed  with  its  axis  at  105°,  there 
would  remain  an  astigmatism  which  might  be  corrected  by 
a  concave  cylinder  with  its  axis  at  about  70°,  and  the  turn- 
ing of  the  cylinder  before  the  eye  10°  in  that  direction 
[to  95°]  would  cause  the  remaining  astigmatism  to  so 
change  that  its  meridians  would  be  at  5°  and  95°,  where  a 
measurement  of  it  would  reveal  the  fact  that  the  cylindri- 
cal lens  employed  was  0.50  D.  too  strong. 

Aberration  and  Irregular  Astigmatism. — The  differ- 
ence in  the  refraction  of  different  parts  of  the  pupil  is  to  be 
ascertained  by  measuring  the  refraction  for  each  part  sep- 
arately, just  as  though  it  were  a  case  of  simple  hyperopia 
or  myopia,  care  being  taken  to  confine  each  observation 
strictly  to  the  little  portion  of  the  pupil  the  refraction  of 
which  it  is  desired  to  ascertain. 

The  amount  of  aberration  or  irregular  astigmatism 
that  is  thus  ascertained  is  of  some  scientific  interest  and 
occasionally  of  practical  importance  as  bearing  on  the 
prognosis  of  conical  cornea,  or  of  the  changes  of  refraction 
in  the  lens  which  precede  cataract. 

Generally,  however,  the  important  practical  point 
about  aberration  or  irregular  astigmatism  is  its  distribution. 
For  practical  purposes,  the  surgeon  desires  to  ascertain 
which  part  of  the  pupil  is  free  from  any  such  defect,  as 
that  part  will  furnish  the  best  visual  zone ;  and  by  what 
lenses  that  visual  zone  can  be    made  most  useful  to  the 


ABERRATION  AND  IRREGULAR  ASTIGMATISM. 


85 


patient.  The  need  for  careful  study  to  develop  these  points 
is  sometimes  great.  Figures  21  and  22  represent  the 
appearances  brought  out  by  thorough  investigation  of  a  case 
of  considerable  astigmatism,  coincident  with  equally  pro- 
nounced positive  aberration.  Without  careful  study  of  the 
visual  zone  at  the  proper  distances,  it  would  have  been  easy 
to  set  the  case  down  as  one  of  aberration,  and  to  have  over- 
looked the  astigmatism  entirely.        If  the  aberration  en- 


FiG.  21. 


Fig.  22. 


croaches  decidedly  upon  the  area  of  the  pupil  as  deter- 
mined by  a  moderate  light,  it  may  be  necessary  to  give  a 
correcting  lens,  for  use  at  near  work  and  on  exposure  to 
bright  light,  different  from  the  one  required  when  the  pupil 
will  be  somewhat  larger.  Or  the  surgeon  may  need  to 
caution  the  patient  that  under  certain  conditions  of  light 
he  must  expect  the  correcting  glasses  to  give  slightly  im- 
perfect vision. 

A  fact  to  be  borne  constantly  in  mind  in  the  applica- 
tion of  skiascopy  is  that  it  is  not  the  high  degrees  of  aber- 
ration and  irregular  astigmatism  that  are  of  most  practical 
importance,  requiring  the  surgeon  to  take  into  account 
their  bad  effects.  More  frequently  it  is  the  slight  imper- 
fections of  this  kind  situated  within  the  portion  of  the 
pupil  used  for  accurate  vision  that  need  to  be  recognized  and 
taken  into  account,  when  prescribing  glasses  or  in  giving 


86  APPLICATION  WITH  THE  PLANE  MIRROR. 

an  opinion  as  to  the  value  of  glasses.  These  low  degrees 
of  imperfection  are  to  be  recognized  and  studied  only  when 
the  surgeon's  eye  is  close  to  the  point  of  reversal  for  the 
visual  zone  after  the  effects  of  hyperopia,  myopia  and  regu- 
lar astigmatism  have  been  excluded  by  placing  the  proper 
glasses  before  the  patient's  eye. 

The  investigation  of  aberration  and  irregular  astigma- 
tism is  the  last  step  in  skiascopy.  A  step  very  frequently 
not  taken,  yet  essential  to  complete  certainty  and  accuracy 
in  the  objective  measurement  of  refraction.  In  a  small  pro- 
portion of  cases,  it  will  lead  to  modification  of  the  glasses 
previously  selected  as  best,  and  in  a  much  larger  proportion 
of  cases  it  will  discriminate  sharply  between  the  lenses 
which  really  best  correct  the  ametropia  and  others  which 
appear  to  give  equally  good,  or  almost  equally  good  sub- 
jective results. 

To  carry  it  to  completion  will  often  require  more  time 
and  effort  than  has  been  necessary  for  all  other  parts  of  the 
skiascopic  examination.  Nor  is  this  strange,  for  it  often 
includes  the  measurement  of  hyperopia  or  myopia  with 
astigmatism  in  two  or  more  areas  of  distinctly,  though 
slightly,  different  refraction.  It  is  not  a  distinct  application 
of  the  test  but  its  application  to  parts  of  the  pupil,  instead 
of  to  the  pupil  as  a  whole.  It  requires  no  special  directions 
and  cannot  be  much  elucidated  by  examples.  It  is  to  be 
mastered  by  a  full  understanding  of  the  optical  principles 
of  the  test.  Chapter  II,  strict  observance  of  the  conditions 
of  accuracy  set  forth  in  Chapter  III,  and  the  exercise  of  the 
needful  care  and  patience. 

Measurement  of  Accommodation. — The  objective 
determination  of  the  nearest  point  for  which  the  eye  can 
be  focused  is  possible  only  by  skiascopy.  It  is  sometimes 
of  importance  as  in  cases  of  suspected  cycloplegia  in  child- 


MEASUREMENT  OF  ACCOMMODATION.  87 

Ten,  or  others  for  whom  the  subjective  test  cannot  be 
relied  on.  In  determining  the  condition  of  the  accommo- 
dation in  an  eye  with  imperfect  vision,  or  in  recognizing 
any  slight  remaining  accommodation  after  the  use  of  a 
mydriatic,  the  test  is  also  of  practical  value. 

To  make  it,  the  surgeon  first  ascertains  the  refraction 
of  the  eye,  and  then  places  before  it  such  lens  or  lenses  as 
will  correct  astigmatism  and  bring  the  point  of  reversal  to 
a  distance  of  one  metre  or  a  little  less.  He  then  places 
himself  at  this  distance  from  the  patient,  and  directs  the 
patient  to  fix  his  gaze  upon  some  object  on  the  farther  side 
of  the  room  in  such  a  position  that  the  visual  axis  of  the 
eye  under  examination  shall  pass  as  close  as  possible  to  the 
surgeon's  eye.  The  point  of  a  finger  or  pencil  is  then  held 
close  to  the  patient's  eye,  about  the  near  limit  of  conver- 
gence and  in  the  visual  axis,  so  that  the  direction  of  the 
visual  axis  shall  not  be  changed  during  the  test. 

The  patient  is  then  directed  to  look  first  at  the  object 
across  the  room,  then  at  the  point  of  the  pencil  close  to  his 
eye.  The  surgeon  by  watching  his  other  eye  can  ascertain 
whether  the  movements  of  convergence  are  really  executed. 
Very  strong  convergence  being  impossible  without  strong 
accommodative  effort,  if  any  power  of  accommodation 
remains  the  eye  will  be  seen  to  grow  more  myopic  when 
the  pencil  is  looked  at,  and  less  so  when  the  distant  object 
is  fixed,  an  inverted  movement  of  the  light  in  the  pupil 
becoming  apparent  in  the  one  position  and  disappearing  in 
the  other. 

To  measure  the  amount  of  accommodation  the  sur- 
geon may  approach  the  observ^ed  e}'e  until  the  point  of 
reversal  is  reached  for  the  eye  during  \'ery  strong  conver- 
gence, or  the  lens  before  the  eye  may  be  modified  in  the 
direction  of  weaker  convex  or  stronger  conca\'e  until  the 


88  APPLICATION  WITH  THE  PLANE  MIRROR. 

point  of  reversal  is  brought  with  a  new  lens,  with  the 
accommodation,  to  the  same  distance  as  it  was  brought  by 
the  original  lens  without  accommodation.  The  difference 
between  the  lenses  in  this  case  representing  the  amount  of 
accommodation  present. 

For  example  :  in  a  case  of  hyperopia  2  D.  to  ascertain 
if  accommodation  was  present  a  convex  3  D.  lens  would  be 
placed  before  the  eye.  This  would  bring  to  one  metre  the 
point  of  reversal  of  the  eye  with  its  accommodation  relaxed. 
The  surgeon  at  a  little  less  than  a  metre  could  get  through 
it  an  erect  movement  of  the  light  in  the  visual  zone  when, 
the  patient  was  looking  across  the  room.  If  now,  on  look- 
ing at  the  point  of  a  finger  held  two  inches  in  front  of  the 
eye,  the  movement  becomes  distinctly  inverted,  the  light 
in  the  pupil  moving  against  the  light  on  the  face,  it  is 
known  that  accommodation  is  present.  In  place  of  the 
3  D.  lens  a  weaker  convex  may  be  substituted,  and  if  the 
strong  convergence  of  the  visual  axis  still  brings  an  in- 
verted movement  of  light  in  the  pupil,  a  still  weaker  lens 
for  this.  In  this  way  if  it  be  found  that  with  the  i  D.  lens 
the  patient  is  able  by  strong  convergence  and  coincident 
accommodative  effort,  to  bring  the  point  of  reversal  to  just 
one  metre,  the  difference  between  the  3  D.  lens  and  the 
I  D.  =2  D.  will  be  the  amount  of  accommodation  pres- 
ent. 

Instead  of  changing  the  lens,  the  surgeon  can  approx- 
imately estimate  the  amount  of  accommodation  by  bring- 
ing his  eye  closer  to  that  of  the  patient  and  finding  the 
new  position  of  the  point  of  reversal  caused  by  the  exer- 
tion of  the  accommodation.  Where  much  accommodation 
is  present,  such  an  approximate  determination  should  first 
be  made,  but  it  is  open  to  the  inaccuracies  attendant  on 
any  skiascopic  determination  at  a  short  distance. 


CHAPTER  VII. 

PRACTICAL    APPLICATION    WITH    THE    CONCAVE    MIRROR. 

The  Source  of  Light. — The  room  is  to  be  thoroughly 
darkened,  and  to  secure  this,  it  is  well  to  have  the  original 
source  of  light  shaded.  This  light  will,  however,  usually 
be  back  of  the  patient  and  except  for  the  determination  of 
the  principal  meridians  of  astigmatism,  the  farther  it  is 
behind  the  patient  the  better,  so  that  it  is  less  essential  to 
have  the  light  thoroughly  shaded.  It  is  also  of  less  import- 
ance that  the  original  source  of  light  should  be  small ; 
still  the  separation  of  light  and  shade  should  be  as  sharp 
as  possible,  so  that  the  opaque  shade  with  an  opening  oppo- 
site the  brightest  part  of  the  flame  will  be  found  service- 
able. The  opening  in  the  shade  may  be  two  or  three  cen- 
timetres in  diameter,  so  long  as  the  original  source  of  light 
is  a  metre  or  more  away  from  the  mirror.  But  when  this 
is  brought  near  the  mirror  to  bring  out  the  band  of  light 
in  astigmatism,  the  opening  should  be  one  centimetre  in 
diameter  or  less. 

The  surgeon  places  himself  a  little  over  one  metre 
from  the  patient.  On  throwing  the  light  upon  the  patient's 
face  with  the  mirror,  it  is  found  that  the  area  of  light 
on  the  face  moves  with  the  mirror  just  as  in  the  case 
of  the  plane  mirror.  The  same  reflections  from  the  cornea 
and  trial  lenses  are  to  be  recognized  and  provided  for,  and 
within  the  pupil  lies  a  similar  portion  of  red  fundus  reflex 
bounded  by  shadow,  which  is  the  subject  of  observation 
7  (89) 


90        APPLICATION  WITH  THE  CONCAVE  MIRROR. 

during  the  test.  If,  however,  the  light  in  the  pupil  be  seen 
to  move  ivith  the  light  on  the  face,  the  eye  is  myopic  more 
than  I  D.  If  the  light  in  the  pupil  be  seen  to  move  against 
the  light  on  the  face,  the  eye  is  hyperopic,  emmetropic,  or 
myopic  less  than  i  D. 

Hyperopia. — If  the  mirror  be  rotated  about  a  vertical 
axis  from  right  to  left,  the  area  of  the  light  in  the  pupil 
w^ill  be  seen  to  move  from  left  to  right,  that  is,  against  the 
mirror  and  against  the  light  on  the  face.  That  this  is  really 
an  erect  movement,  we  know  from  the  demonstrations  as 
to  the  real  direction  of  the  movement  of  the  light  on  the 
retina  in  Chapter  II.  The  difference  between  erect  move- 
ment and  movement  with  the  lighten  the  face  must  be  borne 
in  mind.  With  the  concave  mirror,  the  one  is  just  the 
opposite  of  the  other.  The  movement  with  the  light  on 
the  face  being  an  inverted  movement,  and  the  movement  in 
the  pupil  against  the  light  on  the  face  the  erect  movement. 

As  with  the  plane  mirror,  the  movement  will  be  swift  if 
the  hyperopia  be  of  low  degree,  slower  if  of  higher  amount. 
The  convex  lens  is  now  to  be  placed  before  the  eye,  the 
swiftness  of  the  movement  in  the  pupil  being  the  guide  to 
the  strength  probably  required.  If  the  observer  is  not  able 
to  judge  by  this  movement,  let  him  at  first  employ  in  suc- 
cession lenses  that  differ  considerably  in  strength,  as  the  2, 
4  and  6  D.,  increasing  the  strength  as  long  as  the  move- 
ment in  the  pupil  is  against  the  movement  on  the  face. 

When  a  lens  is  reached  that  causes  movement  of  light 
in  the  pupil  witli  the  light  on  the  face,  slightly  weaker 
lenses  are  to  be  tried  until  two  consecutive  lenses  are  found, 
of  which  one  gives  the  movement  against  the  light  on  the 
face  and  the  next  stronger  causes  movement  with  the  light 
on  the  face.  Between  these  two  lies  the  lens  strength 
which  would  bring  the  point  of  reversal  to  the  surgeon's 


HYPEROPIA.  91 

eye.  This  being  one  metre  from  the  patient,  it  is  the  lens 
which  causes  i  D.  of  myopia  ;  and  by  subtracting  i  D. 
from  its  strength  the  hyperopia  of  the  eye  is  obtained. 

For  example  :  suppose  hyperopia  of  4  D.  The  light 
in  the  pupil  will  move  against  the  light  on  the  face  at  the 
first  inspection,  and  also  with  the  convex  2  D.  and  4  D. 
lenses.  With  the  convex  6  D.  it  is  found  to  move  witlt  the 
light  on  the  face.  On  trying  the  convex  5  D.  the  move- 
ment is  indeterminable.  With  the  4.75  D.  it  is  very  rapid, 
but  still  against  the  light  on  the  face.  With  the  5.25  D. 
it  is  equally  rapid  but  loith  the  light  on  the  face.  The  lens 
strength  between  the  two,  or  the  5  D.,  is  then  the  one  which 
causes  i  D.  of  myopia,  and  5  D.  the  strength  of  the  lens 
minus  i  D.  the  myopia  caused  by  it,  leaves  4  D.  the  lens 
strength  required  to  correct  the  hyperopia  present. 

Myopia. — In  the  mass  of  cases,  inspection  without 
a  lens  will  show  movement  of  light  in  the  pupil  with  the 
movement  of  the  light  on  the  face,  indicating  that  the 
point  of  reversal  is  between  the  surgeon  and  the  patient. 
When  this  is  the  case,  concave  lenses  are  to  be  tried,  their 
strength  being  indicated  by  the  rate  of  movement,  or  if 
this  be  not  a  sufficient  guide,  they  may  be  tried  in  series 
with  an  interval  of  2  D.,  or  more,  until  one  is  found  which 
causes  the  light  in  the  pupil  to  move  against  the  light  on 
the  face. 

As  the  point  of  reversal  is  thus  brought  nearer  to  the 
observer's  eye,  the  light  area  in  the  pupil  becomes  more 
brilliant  and  its  movement  more  rapid.  When  the  lens  has 
been  found  which  causes  the  light  in  the  pupil  to  move 
against  the  light  on  the  face,  slightly  weaker  lenses  are  to 
be  tried  until  it  has  been  certainly  ascertained  which  is  the 
weakest  lens  that  will  cause  the  movement  against  that  of 
the  light  on  the  face,  and  which  is  the  strongest  lens  that 


92       APPLICATION  WITH  THE  CONCAVE  MIRROR. 

still  allows  movement  in  the  pupil  witJi  the  light  on  the 
face.  Between  these  two  lies  the  lens  strength  which  leaves 
the  eye  i  D.  myopic ;  this  lens  strength  added  to  i  D.  will 
give  the  total  myopia  present. 

For  example  :  suppose  the  myopia  present  to  be  8.5  D. 
The  movement  in  the  pupil  without  any  lens  will  be  very 
slow,  and  the  light  area  round  and  dim.  Judging  from  this 
appearance,  the  first  lens  tried  may  be  the  concave  5  D. 
With  it  the  light  in  the  pupil  will  appear  more  brilliant 
and  its  movement  will  be  more  rapid,  but  it  will  still  be 
witJi  the  movement  of  the  light  on  the  face.  Next,  the 
concave  8  D.  will  be  tried.  The  movement  of  light  will 
be  found  still  more  rapid,  but  now  against  that  of  the  light 
on  the  face.  With  the  concave  7  D.  it  will  be  found 
equally  rapid,  but  ii'ttli  the  light  on  the  face.  With  the 
7.5  D.  it  will  not  be  distinguishable.  Hence  the  7.5  D.  lens 
leaves  i  D.  of  myopia  still  uncorrected,  and  added  to  that 
I  D.,  gives  8.5  D.  the  total  myopia  present. 

If  the  myopia  be  of  low  degree,  the  test  without  a 
lens  will  show  either  no  distinguishable  movement  of  the 
light  in  the  pupil  [for  myopia  of  i  D.]  or  movement  in 
the  pupil  against  the  movement  of  the  light  on  the  face 
[for  myopia  of  less  than  i  D.] .  In  the  former  case,  the 
test  is  to  be  repeated  with  very  weak  convex  and  concave 
lenses  [0.25  D.  or  0.50  D.].  The  convex  will  give  a 
movement  of  the  light  in  the  pupil  luith  the  light  on  the 
face,  and  the  concave  a  movement  against  the  light  on  the 
face. 

If  the  movement  is  found  to  be  against  the  light  on 
the  face  to  start  with,  the  convex  lenses  are  to  be  tried,  com- 
mencing with  I  D.  lens  which  will  cause  the  movement 
luith  the  light  on  the  face,  and  will  show,  therefore,  that 
the    refraction    is    myopia    and    not    emmetropia,    or    low 


MYOPIA.  93 

hyperopia.  The  weaker  lenses  are  then  to  be  tried  and  the 
one  which  causes  i  D.  of  myopia  thus  ascertained.  Since 
this  lens  is  added  to  the  myopia  of  the  eye  to  cause  i  D. 
of  myopia,  it  must  be  subtracted  from  i  D.  to  find  the 
amount  of  myopia  originally  in  the  eye  ;  the  difference 
between  it  and  i  D.  being  the  myopia  present. 

Thus,  in  a  case  of  myopia  of  0.50  D.,  the  light  will  be 
found  to  move  against  the  light  on  the  face,  without  any 
lens  or  with  a  0.25  D.  convex.  But  it  will  be  found  to 
move  ivitli  the  light  on  the  face  with  a  convex  i  D.  or 
0.75  D.,  and  with  an  0.50  D.  the  movement  should  be  in- 
distinguishable. The  convex  0.50  D.  then,  causes  i  D.  of 
myopia  ;  and  subtracting  it  from  i  D.,  leaves  0.50  D.  the 
degree  of  myopia  existing  in  the  eye. 

Emmetropia. — In  emmetropia,  on  the  first  trial,  the 
light  in  the  pupil  is  found  to  move  against  the  light  on  the 
face,  and  rapidly.  With  convex  lenses  it  is  found  that 
the  0.75  D.,  or  anything  weaker  still  allows  this  movement 
against  the  mirror;  but  the  1.25  D.  or  anything  stronger 
causes  motion  in  the  pupil  with  the  light  on  the  face  ;  and 
that  the  convex  i  D.  causes  no  perceptible  movement. 
Hence,  the  convex  i  D.  lens  causing  i  D.  of  myopia,  the 
eye  without  a  lens  must  be  emmetropic. 

Regular  Astigmatism. — The  test  beginning  as  for 
simple  hyperopia  or  myopia,  as  the  point  of  reversal  for 
one  of  the  principal  meridians  is  brought  near  the  ob- 
server's eye,  the  movement  of  light  becomes  notably  more 
rapid  in  one  meridian  than  in  the  other,  indicating  the 
presence  of  this  form  of  ametropia.  When  this  is  recog- 
nized, the  lenses  used  are  to  be  such  as  give  a  movement 
of  the  light  in  the  pupil  with  the  light  on  the  face  in  all 
meridians.  Thus,  if  the  eye  has  been  hyperopic,  the  con- 
vex lenses  used  before  the  eye  must  be  increased  in  strength 


94        APPLICATION  WITH  THE  CONCAVE  MIRROR. 

until  the  movement  ivith  the  light  on  the  face  occurs  in  all 
directions.  Or,  if  the  eye  is  myopic,  the  increase  of 
strength  in  the  concave  lenses  must  stop  so  soon  as  any 
movement  is  seen  in  the  pupil  against  the  movement  of 
ligfht  on  the  face.  And  the  lens  which  causes  this  must  be 
replaced  by  a  weaker  one  that  just  allows  movement  with 
the  light  on  the  face  in  all  meridians. 

The  lens  aimed  at  is  the  one  which  will  bring  the 
point  of  reversal  for  the  least  myopic  meridian,  just  to  the 
surgeon's  eye,  i  metre  from  the  patient.  If  this  is  exactly 
attained,  there  will  be  in  that  meridian  no  perceptible 
movement  of  light  and  shadow,  but  the  movement  in  the 
other  princij^al  meridian  will  still  be  with  that  of  the  light 
on  the  face. 

When  this  lens  has  been  found,  the  original  source  of 
light,  which,  up  to  this  time  has  been  kept  at  as  great  a 
distance  as  possible  from  the  mirror,  is  to  be  brought  closer 
to  the  mirror,  so  that  the  image  of  it  formed  at  the  conju- 
gate focus  in  front  of  the  mirror  will  be  removed  farther 
from  the  mirror  and  closer  to  the  patient's  eye. 

The  lens  before  the  patient's  eye  brings  the  point  of 
reversal  for  the  least  myopic  meridian  to  the  eye  of  the  sur- 
geon and  necessarily  places  the  point  of  reversal  for  the 
more  myopic  meridian  somewhere  between  the  surgeon  and 
patient.  The  object  of  bringing  the  original  source  of 
light  nearer  to  the  mirror  is  to  carry  the  immediate  source 
of  light  to  the  point  of  reversal  for  this  more  myopic 
meridian.  As  the  light  approaches  its  proper  position,  the 
area  of  light  in  the  pupil  becomes  more  and  more  band-like 
in  form,  being  most  distinctly  so  when  the  immediate 
source  of  light  corresponds  with  the  point  of  reversal  for 
the  more  myopic  meridian. 

When  this  "is  attained,  the  direction  of  the  band  is  to 


REGULAR    ASTIGMATISM.  95 

be  carefully  noted  as  indicating  the  direction  of  the  princi- 
pal meridian  of  least  myopia.  This  direction  having 
been  determined  and  recorded,  the  original  source  of  light 
is  again  moved  as  far  away  from  the  mirror  as  possible,  and 
measurement  of  the  refraction  in  the  least  myopic  meridian 
completed  as  for  a  case  of  simple  myopia. 

Then,  the  lenses  are  so  changed  as  to  bring  the  point 
of  reversal  for  the  more  myopic  meridian  to  the  surgeon's 
eye  i  metre  distant  from  the  patient :  and  the  lens  that  is 
found  to  do  this,  shows  by  the  addition  of  i  D.  to  a  con- 
cave, or  the  subtraction  of  i  D.  from  the  strength  if 
convex,  the  amount  of  myopia  or  hyperopia  in  the  second 
meridian.  The  difference  between  the  two  meridians  is 
the  amount  of  astigmatism. 

When  it  has  thus  been  ascertained,  the  cylindrical 
lens  correcting  it  is  to  be  placed  before  the  eye  and  with  it, 
the  spherical  lens,  which  will  bring  the  point  of  reversal  to 
a  distance  of  one  metre.  The  trial  is  then  repeated  and  if 
the  point  of  reversal  be  found  at  the  surgeon's  eye  for  all 
meridians  of  the  pupil,  the  determination  already  made  is 
accurate.  If,  however,  there  be  found  distinct  movement 
in  the  visual  zone  in  some  one  direction,  while  movement 
in  the  principal  meridian  perpendicular  thereto  is  abolished, 
the  cylinder  selected  does  not  perfectly  correct  the  astig- 
matism. 

If  this  movement  be  in  one  of  the  principal  meridians 
as  previously  determined,  [in  the  direction  of  the  axis  of 
the  cylinder  placed  before  the  eye,  or  at  right  angles  to 
that  axis]  the  cylinder  has  been  properly  placed,  but  is  too 
strong  or  too  weak,  and  its  strength  must  be  diminished  or 
increased  according  to  the  indications  of  the  movement.  If, 
however,  the  movement  appears  to  be  in  a  meridian  different 
from  either  of  the  principal  meridians  as  at  first  determined, 


96        APPLICATION  WITH  THE  CONCAVE  MIRROR. 

[different  from  the  direction  of  the  axis  of  the  cylindrical 
lens  before  the  eye,  or  the  principal  meridian  at  right 
angles  to  that  axis]  the  axis  has  not  been  properly  placed 
— does  not  conform  exactly  to  the  direction  of  the  princi- 
pal meridian. 

If  the  cylindrical  lens  before  the  eye  is  of  the  right 
strength  or  too  weak,  its  axis  needs  to  be  turned  slightly 
toward  the  axis  of  a  similar  cylinder,  which  will  correct 
the  remaining  astigmatism.  If  the  cylindrical  lens  already 
before  the  eye  is  too  strong,  its  axis  needs  to  be  turned 
toward  the  proper  position  the  axis  for  a  cylindrical  lens  of 
the  opposite  kind  that  would  correct  the  astigmatism. 
Such  a  change  in  the  direction  of  the  axis  of  the  cylinder 
is  to  be  made,  and  the  test  repeated  until  the  correction  of 
any  remaining  astigmatism  conforms  exactly  with  the 
direction  of  the  lens  before  the  eye.  This  remaining 
astigmatism  must  be  corrected  by  a  change  in  the  strength 
of  the  lenses  employed. 

For  example  :  suppose  an  eye  to  have  compound  hy- 
peropic  astigmatism  corrected  by  +4  sph.  O  +2  cyl.  axis 
90°.  The  first  inspection  of  the  pupil  shows  the  light 
moving  against  the  light  on  the  face  in  all  meridians. 
Convex  lenses  2  D.  and  4  D.  placed  before  the  eye  show  the 
same  thing.  Convex  6  D.  shows  the  light  moving  against 
the  light  on  the  face  from  side  to  side,  but  ivith  it  in  a  ver- 
tical direction.  It  thus  becomes  evident  that  astigmatism 
is  present.  Still  stronger  convex  lenses  are  to  be  tried. 
The  8  D.  lens  shows  movement  in  the  pupil  with  the  light 
on  the  face  in  all  meridians.  The  7  D.  lens  shows  move- 
ment very  indefinite  or  indistinguishable  in  the  horizontal 
meridian,  but  clearly  with  the  light  on  the  face  in  the  ver- 
tical meridian.  This  lens  then,  brings  the  point  of  rever- 
sal for  the  less  myopic  [more  hyperopic  without  the  lens] 
meridian  to  the  surgeon's  eye. 


REGULAR  ASTIGMATISM.  97 

The  next  step  is  to  bring  the  original  source  of  light 
closer  to  the  mirror  so  as  to  cause  the  immediate  source  of 
light  to  fall  at  the  point  of  reversal  for  the  more  myopic 
[less  hyperopic]  meridian,  which  will  now  be  one-third  of 
a  metre  from  the  patient's  eye.  To  do  this,  [supposing 
that  the  mirror  has  a  focal  distance  of  one-quarter  of  a 
metre,  ten  inches]  it  will  be  necessary  to  bring  the  source 
of  light  to  within  two-fifths  of  a  metre  of  the  mirror.  That 
is,  the  immediate  source  of  light  to  be  at  one-third  of  a 
metre  from  the  patient,  must  be  at  two-thirds  of  a  metre 
from  the  mirror  corresponding  with  1.5  D.  of  focusing 
power.  The  total  focusing  power  of  the  mirror  being  equal 
to  4  D.,  the  light  must  be  so  placed  that  the  divergence  of 
its  rays  will  correspond  to  4-1.5^2.5  D.  That  is,  the 
light  must  be  two-fifths  of  a  metre  from  the  mirror.  When 
the  light  is  in  this  position,  the  area  of  light  in  the  pupil 
will  assume  the  most  distinct  band-like  appearance,  run- 
ning in  the  direction  of  the  principal  meridian  of  least 
myopia  [greatest  h^^peropia]  in  this  case  horizontal. 

Having  thus  determined  the  direction  of  the  principal 
meridians,  one  being  known  from  the  direction  of  the  other, 
the  original  source  of  light  is  again  placed  back  of  the 
patient  as  far  as  possible,  and  the  refraction  in  the  horizon- 
tal meridian  carefully  tested  by  trying  first  the  +6.5  D. 
spherical  lens,  and  then  the  +7.5  D.  spherical  lens  before 
the  eye,  the  former  of  which  shows  the  movement  in  a 
horizontal  meridian  against  the  light  on  the  face,  and  the 
latter  a  movement  in  the  same  meridian  luith  the  light  on 
the  face,  thus  fixing  the  refraction  of  that  meridian  as  7  D. 
— I  D.  =6.  D  of  hyperopia. 

Weaker  convex  lenses  are  then  to  be  tried  until  it  is 
found  that  with  the  5.5  D.,  the  light  moves  ivith  the  light 
on  the  face  in  the  vertical  meridian,  and  with  the  4.5  D.  it 


98        APPLICATION  WITH  THE  CONCAVE  MIRROR. 

moves  against  the  light  on  the  face  in  the  vertical  meridian, 
while  the  5  D.  gives  no  distinguishable  movement  in  that 
meridian,  showing  that  5  D. — i  D.=4  D.  is  the  amount  of 
hyperopia  in  the  less  hyperopic  meridian.  The  difference 
between  the  two  then  is  found  to  be  2  D.,  the  amount  of 
regular  astigmatism  present. 

The  surgeon  will  then  place  before  the  patient  convex 
5  D.  spherical  with  convex  2  D,  cylindrical  axis  vertical  ; 
and  on  again  trying  the  test,  will  find  that  he  is  at  the 
point  of  reversal  for  all  meridians.  But  if  on  placing  the 
cylindrical  lens  he  makes  a  slight  error  in  the  direction  of 
its  axis,  placing  it  say  at  5°  one  side  from  the  vertical,  he 
will  find  on  testing  the  eye  some  appearance  of  astigmatism 
with  its  axis  inclined  several  degrees  in  the  other  direction 
from  the  vertical.  And,  to  get  rid  of  this  astigmatism, 
he  has  to  move  the  axis  of  the  cylindrical  lens  to  its  proper 
position,  pushing  it  towards  the  axis  of  a  convex  cylinder 
that  would  be  required  to  correct  this  remaining  astigma- 
tism. 

If  the  case  be  one  of  slightly  myopic  or  high  mixed 
astigmatism,  the  first  inspection  may  show  a  movement 
witJi  the  light  on  the  face  in  one  direction,  while  the  move- 
ment is  against  the  light  on  the  face  in  the  other  meridian. 
This,  of  course,  will  indicate  at  once  the  presence  of  astig- 
matism. The  fact  that  it  may  occur  makes  it  important 
that  the  first  observation  on  the  pupillary  movements 
should  include  the  movements  in  different  meridians. 

With  the  concave  mirror  [the  immediate  source  of 
light  necessarily  lying  as  far  in  front  of  the  mirror  as  its 
principal  focus  or  even  farther]  if  the  astigmatism  be  of 
quite  low  degree,  when  the  least  myopic  point  of  reversal 
is  at  the  surgeon's  eye,  the  more  myopic  point  of  reversal 
will  be  at  the  immediate  source  of  light,  or  even  closer  to 


REGULAR  ASTIGMATISM.  99 

the  mirror  without  any  change  in  the  position  of  the  origi- 
nal source.  Thus  the  most  distinct  band-like  appearance 
of  the  light  in  the  pupil,  the  clearest  difference  between 
the  movement  against  the  light  on  the  face  in  one  meridian 
and  the  indefinite  movement  in  the  other  meridian  will  be 
attained  without  bringing  the  original  source  of  light  any 
nearer  to  the  mirror  than  its  usual  position.  This  must  be 
borne  in  mind  for  low  degrees  of  astigmatism. 

Aberration  and  Irregular  Astigmatism. — With  the 
concave  mirror  and  the  need  of  bringing  the  point  of 
reversal  to  a  fixed  distance  from  the  patient's  eye,  the 
measurement  of  the  amount  of  aberration  and  irregular 
astigmatism  becomes  very  much  more  tedious  and  difficult 
than  with  the  plane  mirror,  though  not  impossible.  It  is, 
however,  not  difficult  to  detect  the  presence  of  such  defects, 
and  to  ascertain  which  portion  of  the  pupil  they  occupy, 
and  which  portions  being  comparatively  free  from  them  are 
available  as  a  visual  zone.  As  to  the  importance  of  such  a 
study  of  the  pupil,  what  has  been  said  in  the  chapter  on 
the  plane  mirror  will  equally  apply  here. 

Measurement  of  Accommodation. — With  the  aid  of 
lenses,  usually  concaves,  the  near  point  of  accommodation 
may  be  brought  to  the  required  distance  of  one  metre  from 
the  eye  and  the  amount  of  accommodation  thus  measured. 
The  arrangement  of  the  patient's  and  surgeon's  eyes,  and 
of  the  points  to  be  looked  at  is  the  same  as  that  described 
in  connection  with  the  measurement  of  accommodation  with 
the  plane  mirror.  It  is,  of  course,  impossible  to  make  the 
approximate  determination  of  the  accommodation  with  the 
concave  mirror  by  the  surgeon  approaching  the  eye  of  the 
patient.  He  must  rely  on  a  change  of  lenses  to  bring  the 
point  of  reversal  to  the  fixed  distance  of  one  metre. 


CHAPTER  VIII. 

GENERAL    CONSIDERATIONS. 

Apparatus. — In  the  chapter  of  the  Conditions  of  Accu- 
racy, something  has  already  been  said  as  to  the  apparatus 
by  which  these  conditions  are  best  complied  with.  The 
requirements,  to  meet  which  the  apparatus  for  skiascopy 
is  to  be  adapted,  are  that  it  shall  furnish  the  conditions 
necessary  to  the  greatest  accuracy,  and  that  it  shall  facili- 
tate the  finding  of  the  lens  that  will  bring  the  point  of 
reversal  to  the  surgeon's  eye. 

The  Mirror. — As  has  been  stated,  the  essential  point, 
in-  the  mirror  is  to  have  the  sight  hole  free  from  reflections. 
This  may  be  obtained  by  having  the  glass  thin  if  the  sight 
hole  is  cut  through  it,  having  its  margin  free  from  chip- 
ping, beveled  as  little  as  possible,  and  thoroughly  blackened 
with  a  dead  black. 

If  the  sight  hole  is  not  cut  through  the  glass,  but  is 
merely  an  aperture  in  the  silvering,  the  glass  may  be  much 
thicker  and  there  is  no  ground  glass  to  deal  with.  The 
difficulty  with  such  a  mirror  is  in  keeping  the  exposed 
glass  at  the  sight  hole  clean.  Unless  great  care  is  taken 
in  preserving  it  from  dust,  and  care  in  removing  any  that 
falls  upon  it,  there  will  be  a  ring  of  dust  in  the  periphery 
of  the  sight  hole,  which  will  reflect  more  light  than  would 
the  ground  glass.  And,  it  is  difficult  to  keep  this  space 
entirely  clean  without  chipping  into  the  backing  of  the  mir- 
ror in  such  a  way  as  to  cause  annoving  reflections.       But, 

(100) 


THE  MIRROR.  101 

however  difficult,  it  is  essential  to  have  the  sight  hole  free 
from  reflections. 

The  size  of  the  mirror  will  depend  somewhat  upon 
the  purpose  for  which  skiascopy  is  to  be  used.  If  the  mir- 
ror is  to  be  employed  to  measure  refraction  of  all  kinds ;  to 
show  the  movement  of  light  in  the  pupil  with  high  uncor- 
rected hyperopia  or  myopia,  it  must  be  large ;  to  give  the 
range  of  movement  for  the  immediate  source  of  light  that 
is  necessary  to  render  evident  the  direction  of  movement 
in  the  pupil,  when  that  movement  is  slow  and  the  illumi- 
nation of  the  light  area  is  comparatively  feeble. 

The  disadvantage  of  a  large  mirror  is  that  it  gives  a 
large  area  of  light  on  the  face,  especially  when,  as  with  the 
plane  mirror,  the  original  source  of  light  is  brought  close 
to  it.  And  in  this  large  area  of  light  on  the  face  only  the 
light  reflected  by  a  small  portion  of  the  mirror  immediately 
surrounding  the  sight  hole  is  of  any  use  when  the  point  of 
reversal  is  near  to  the  surgeon's  eye  [see  page  30  for  discus- 
sion of  limits  of  the  part  of  the  retina  visible  in  the  pupil] . 
With  a  small  mirror,  making  a  small  area  of  light  on  the 
face,  it  is  easier  to  keep  this  upon  the  eye  than  it  is  to  keep 
properly  directed  the  similarly  limited  portion  of  a  largearea. 

On  this  account,  where  skiascopy  is  used  after  an  ap- 
proximate estimate  of  the  refraction  has  been  made  by  the 
ophthalmoscope  or  other  means,  quite  a  small  mirror  is 
found  convenient.  By  a  large  mirror  is  meant  one  from 
35  to  50  mm.  in  diameter.  By  a  small  mirror  is  meant  one 
under  20  mm.  in  diameter.  The  mirror,  or  at  least  the 
opaque  back  that  carries  it  cannot  be  well  reduced  to  less 
than  20  or  25  mm.,  because,  if  smaller  than  this,  it  w^ill 
admit  light  to  the  e}'e  from  the  original  source  through  the 
space  around  the  mirror,  and  such  light,  though  not  so 
annoving  as  a  reflection  at  the  sight  hole,  is  a  serious  hin- 


102 


GENERAL  CONSIDERATIONS. 


derance  in  the  application  of  the  test.       The  mirror  plate 
then,  must  be  large  enough  to  shade  the  eye. 

A  large  mirror  having  a  metal  cap  with  an  aperture  of 
from  lo  to  15  mm.  in  diameter,  that  can  be  slipped  before 
the  face  of  the  mirror  or  turned  back  at  pleasure,  will  an- 
swer for  all  sorts  of  testing.  Such  a  mirror-  is  shown  in 
figure  23.  As  already  indicated  in  Chapter  III,  the  sight 
hole  should  be  from  2  to  3  mm.  in  diameter. 


The  handle  of  the  mirror  should  be  rather  broad,  so 
that  a  ver>'  slow,  even  rotation  can  be  secured  ;  for  as  the 
point  of  reversal  is  approached,  the  magnified  movement  in 
the  pupil  becomes  so  rapid  that  only  by  moving  the  mirror 
slower,  and  making  excursions  of  very  slight  extent  can 
this  apparent  motion  in  the  pupil  be  readily  followed. 
This  difficulty  of  causing  the  immediate  source  of  light  to 
move  slowly  enough  is  diminished  in  proportion  as  the 
immediate  source  of  light  is  brought  closer  to  the  mirror. 

The  Shade. — The  shade  that  covers  the  original  source 
of  light  should  extend  far  enough  above  and  below  to  pre- 
vent the  escape  of  any  considerable  amount  of  light  into 
the  room.     Where  an  argand  burner  is  used  as  the  source, 

"^  Made  at  my  suggestion  by  Wall  and  Ochs  of  Philadelphia.  Another  form 
is  described  by  Dr.  James  Thorington,  Philadelphia  Polyclinic,  1893,  page 
329. 


THE  SHADE.  103 

a  cylindrical  shade  is  needed,  20  to  25  cm.  long,  and  having 
a  diameter  of  6  or  6.5  cm,,  slightly  greater  than  that  of  the 
chimney  used  so  as  to  allow  a  free  current  of  air  between 
the  shade  and  chimney,  and  thus  diminish  the  heat  from  the 
flame.  An  asbestos  shade  as  proposed  by  Dr.  J.  Thoring- 
ton  (Ann.  of  Ophthalmology  and  Otology,  1895,  p.  5)  is  to  be 
preferred  to  metal  on  account  of  intercepting  the  heat  of 
the  flame. 

The  aperture  of  about  5  mm.,  or  larger,  if  the  test  is 
not  intended  to  be  very  accurate,  should  be  opposite  the 
brightest  part  of  the  flame,  which  ought  to  be  broad  enough 
to  allow  of  slight  change  of  position  of  the  surgeon  with 
reference  to  it  without  its  becoming  hidden  by  the  shade. 

The  Lenses. — Ordinarily  these  are  taken  from  the 
trial  case  and  placed  in  a  trial  frame  before  the  eye.  It  is 
important  to  have  them  clean  and  comparatively  undam- 
aged by  scratching.  The  trial  frames  should  be  such  as  to 
support  the  lenses  well  up  before  the  eye  and  with  their 
centres  before  the  centres  of  the  pupils.  They  must  also 
be  far  enough  away  from  the  face  to  escape  the  touching 
of  the  lashes,  and  to  prevent  the  condensation  of  moisture 
upon  them.  The  interruption  of  the  red  reflex  from  the 
pupil  by  such  an  occurrence  prevents  the  satisfactory  ap- 
plication of  the  test,  and  may  be  quite  puzzling,  because 
the  reason  for  the  obscuration  is  not  immediately  apparent, 
and  it  may  be  ascribed  to  opacities  within  the  eye. 

Support  of  Lenses. — The  trial  frames  have  the  advan- 
tage over  other  supports  for  lenses  to  be  presently  men- 
tioned, that  they  keep  a  constant  position  with  reference  to 
the  patient,  so  that  a  slight  movement  of  the  patient's 
head  does  not  carry  his  eye  away  from  the  centre  of  the 
lens  to  its  periphery  or  beyond. 

When   the  surgeon    has  learned   to  estimate   by  the 


104 


GENERAL  CONSIDERATIONS. 


rapidity  of  movement  of  the  light  in  the  pupil  the  amount  of 
ametropia  remaining  uncorrected,  by  following  the  plan 
here  laid  down  of  considerable  intervals  between  the  lenses 
until  an  approximation  of  the  required  lens  has  been  made, 
the  number  of  changes  of  lenses  for  any  case  is  not  neces- 
sarily great.  So  that  for  any  one  w^ho  does  not  employ 
skiascopy  on  large 'numbers  of  patients  daily,  the  trial 
frame  and  lenses  will  be  found  satisfactory. 

Special  series  of  lenses  mounted  in  revolving  disks 
have  been  arranged  by  Haines  {Ophthalmic  Review,  1886,  p. 
282),  Doyne,  Couper,  Burnett  {Trans.  Am.  Ophthalmol.  Soc, 
1888,  p.  223),  Wurdemann  and  others,  to  save 
time  by  facilitating  the  changes  to  the  lens 
required.  Some  of  these  have  been  designed 
for  the  patient  to  make  the  change  of  lens 
under  direction  of  the  surgeon,  and  others  to 
give  the  surgeon  himself  constant  control  of 
their  movements. 

One  of  the  simplest  arrangements  is  the 
"  hand-skiascope  "  of  Wurdemann  {American 
Journal  of  Ophthalmologij,  1891,  page  223) 
shown  in  figure  24.  The  lenses  are  inserted 
in  a  sheet  of  hard  rubber  which  the  patient 
holds  by  the  handle,  bringing  before  his  eye 
the  lens  the  surgeon  may  indicate. 

In  an  instrument  suggested  bv  the  writer 
the  disk  is  rotated  by  a  rod  one  metre  long  and 
|i:||U  attached  by  a  universal  joint  so  that  it  drops 

out  of  the  way  wdien  not  in  use. 

The  lens  series  runs  from  7  concave  to  7 
convex  spherical  with  0.5  D.  intervals,  requir- 
FiG.  24.       -^g^  ^Q  ^g  supplemented  by  lenses  in  the  trial 
frame  for  high  hyperopia  and  myopia,  or  astigmatism. 


SUPPORT  OF  LENSES.  105 

An  ingenious  piece  of  apparatus  having  a  complete 
series  of  lenses,  both  spherical  and  cylindrical,  arranged  for 
the  purpose  has  been  devised  by  Lambert  (Traiis.  Amer. 
Ophthalmol.  Soc,  1894,  p.  196).  It  has  the  lenses  arranged 
in  two  disks  for  the  special  lenses,  and  detachable  slides 
for  the  cylinders,  enabling  the  surgeon  to  reach  the 
lenses  wanted  quickly.  To  be  compelled  to  run  over  the 
whole  lens  series  to  find  the  one  sought,  would  be  a  way 
of  consuming  time  rather  than  saving  it.  A  series  sufficient 
for  the  approximate  testing  of  the  majority  of  eyes  may 
save  time  where  many  are  to  be  tested,  especially  if  the 
conca^'e  mirror  be  employed. 

Meridian  Indicators. — In  working  with  lenses  in  the 
graduated  trial  frame  one  may  refer  to  its  graduation  to  as- 
certain the  direction  of  the  bands  of  astigmatism.  But 
in  the  darkened  room  this  is  not  convenient.  To  meet 
this  want,  Thorington  (Medical  News,  March  3rd,  1894) 
and  Prince  (Ophthalmic  Review,  July,  1894)  have  suggested 
disks  specially  graduated  for  the  purpose.  The  former, 
figure  25,  called  an  axonometer ;  the  latter,  figure  26,  called 
an  inclinometer. 


Fig.  25.  Fig.  26. 

A  Distance  Measure. — Where  the   concave  mirror  is 
employed,  the  distance  remaining  fixed  throughout  the  test, 


106  GENERAL  CONSIDERATIONS. 

it  is  only  necessary  that  the  surgeon  should  properly  place 
himself  at  the  beginning,  and  retain  his  position.  He  can 
then  dismiss  the  consideration  of  the  distance,  and  consider 
simply  the  changes  made  in  the  lens  before  the  eye. 

With  the  plane  mirror,  no  measure  is  necessary  where 
the  test  is  used  only  to  approximate  the  refraction,  the  sur- 
geon soon  learning  to  guess  at  the  distance  close  enough  to 
be  within  0.25  D.  of  the  amount  of  myopia  present  with 
the  lens  fixed  upon.  But,  for  exact  measurement,  it  is  con- 
venient to  have  something  to  measure  from  the  patient's 
eye  to  the  surgeon's.  This  may  be  either  a  tape  attached 
to  the  trial  frame  or  lens  disk  (Burnett),  picked  up  and 
held  to  the  surgeon's  eye  when  the  test  is  completed,  or  the 
ordinary  metre  stick.  In  either  case,  it  is  convenient  to 
have  the  measure  graduated  in  dioptric  focal  lengtlis  de- 
scribed by  the  waiter  in  the  Mcdic(d  Neivs,  June  27,  1885. 
The  graduation  should  begin  from  the  end  that  is  applied 
to  the  patient's  eye. 

Mydriatics. — In  making  any  measurement  that  is  to 
have  positive  significance,  the  first  essential  is  that  the 
quantity  to  be  measured  should  be  fixed.  When  the  refrac- 
tion of  the  eye  varies  from  moment  to  moment,  it  is  impos- 
sible to  make  a  valuable  measurement  of  it  by  any  method. 
When  it  is  liable  to  vary  from  moment  to  moment,  there  is 
a  liability  to  error,  due  to  such  variations.  Believing  that 
when  consulted  as  to  an  error  of  refraction  or  its  effects,  the 
ophthalmic  surgeon  should  ascertain  its  degree  with  exact- 
ness and  certainty,  the  writer  is  accustomed  to  employ  a 
mydriatic  so  as  to  secure  complete  paralysis  of  accommoda- 
tion in  the  great  majority  of  cases  under  50  years  of  age. 

While  any  of  the  true  mydriatics,  atropin,  daturin, 
duboisin,  hyoscyamin,  or  scopolamin,  will  give  a  satisfac- 
tory paralysis  of  accommodation,  if  a  mydriatic  is  used  solely 


MYDRIATICS.  107 

for  diagnostic  purposes,  homatropin  should  be  selected  on 
account  of  its  briefer  period  of  recover}^  Properly  used,  it 
is  for  practical  purposes  of  diagnosis,  as  reliable  as  any 
mydriatic  we  possess.  To  secure  paralysis  of  accommoda- 
tion, therefore,  four  to  six  drops  of  a  two  to  four  per  cent, 
solution  of  homatropine  hydrobromate  are  to  be  instilled, 
one  drop  at  a  time  at  interv^als  of  five  minutes,  about  an 
hour  before  the  test  is  to  be  applied. 

To  apply  skiascopy  with  the  greatest  ease,  requires  a 
pupil  moderately  dilated.  Like  other  methods  for  the 
measurement  of  refraction,  it  will  not  give  as  accurate  re- 
sults if  the  pupil  be  narrow  ;  and,  on  account  of  the  aber- 
ration and  irregular  astigmatism  that  usually  exist  near  the 
margin  of  the  lens  and  cornea,  the  wide  dilatation  of  the 
pupil  introduces  factors  of  confusion.  The  need,  then,  for 
a  dilated  pupil  is  about  the  same  with  skiascopy  as  for  the 
use  of  the  refraction  ophthalmoscope  or  the  test  lenses,  ex- 
cept that  skiascopy  is  slightly  more  at  a  disadvantage  when 
the  pupil  in  the  dark  room  is  less  than  four  millimetres  in 
diameter.  When  this  is  the  case,  sufficient  dilatation  can 
be  obtained,  with  the  least  inconvenience  to  the  patient,  by 
placing  in  the  eye  a  drop  of  a  two  or  four  per  cent,  solution 
of  cocaine,  thirty  to  fifty  minutes  before  using  the  test. 

Relative  Advantages  of  Plane  and  Concave  Mirrors. 
— The  difference  in  methods  of  using  most  efficiently  the 
plane  and  concave  mirrors  have  caused  most  surgeons  to 
habitually  employ  the  one  or  the  other,  and  to  depend  upon 
it  almost  entirely  for  practical  purposes.  Either,  properly 
used,  will  meet  the  requirements  of  practice. 

In  Astigmatism,  the  plane  mirror  is  capable  of  de- 
termining with  greatest  accuracy  the  meridian  of  greatest 
myopia,  but  not  the  meridian  of  least  myopia.  On  the 
other  hand,  the  concave  mirror  fixes  with  greatest  accuracy 


108  GENERAL  CONSIDERATIONS. 

the  meridian  of  least  myopia,  but  not  that  of  greatest  myopia. 
But,  for  regular  astigmatism — the  astigmatism  that  can 
be  fully  corrected  by  a  cylindrical  lens,  or  by  any  com- 
bination of  cylindrical  lenses — the  principal  meridians  are 
always  perpendicular  to  each  other,  so  that  for  practical 
purposes,  it  is  only  necessary  to  accurately  locate  one  of 
them  ;  and  it  is  a  matter  of  indifference  which  one  this 
shall  be. 

In  positive  aberration^  the  focusing  of  the  light  upon  the 
retina  being  such  that  the  light  area  has  the  sharpest  out- 
line when  the  immediate  source  of  light  is  closer  to  the  eye 
than  the  point  of  reversal,  this  can  only  be  effected  by  the 
concave  mirror,  which,  therefore,  has  so  much  advantage 
over  the  plane  mirror.  It  is  of  some  practical  importance 
in  a  few  cases,  in  which  the  aberration  invades  the  visual 
zone. 

With  negative  aberration,  the  advantage  lies  with  the 
plane  mirror,  but  is  of  still  less  practical  importance  on  ac- 
count of  the  smaller  number  of  cases  of  aberration  of  this 
kind. 

With  a  concave  mirror,  the  distance  from  which  it  can 
be  used  with  advantage  is  fixed,  and  the  surgeon  being  able 
to  readily  check  his  position,  by  a  mark  on  the  neighboring 
wall  or  some  similar  device,  there  is  no  need  of  the  meas- 
urement of  the  distance  between  the  surgeon  and  the 
patient ;  but  all  changes  of  the  movement  of  the  light  in 
the  pupil  must  be  effected  by  a  change  of  the  lens  before 
the  eye.  On  the  other  hand,  the  plane  mirror  can  be  used 
from  any  fixed  distance,  but  allows  a  variation  of  the  dis- 
tance of  the  surgeon  from  the  patient,  and  therefore  requires 
the  fewer  changes  of  the  lens  before  the  eye. 

This  latter  advantage  of  the  plane  mirror  over  the  con- 
cave mirror  may  not  seem  very  great,  but  when  it  comes  to 


ADVANTAGES  OF  PLANE  AND  CONCAVE  MIRRORS.  109 

the  accurate  determination  of  the  refraction,  requiring  re- 
peated inspections  of  the  light  movement  in  the  pupil  from 
within  and  from  beyond  the  point  of  reversal,  it  is  really 
quite  important.  The  disadvantage  of  the  concave  mirror 
may  be  lessened  by  the  employment  of  some  such  arrange- 
ment of  lenses  in  a  disk  as  has  been  referred  to  in  a  preced- 
ing section.  But,  even  in  this  case,  the  fact  that  there  is  a 
complete  break  between  the  appearances  presented  by  one 
lens,  and  the  appearances  present  by  the  use  of  the  lens  next 
stronger  or  weaker,  makes  the  information  obtained  less 
valuable  and  satisfactory  than  that  derived  by  the  move- 
ment of  the  surgeon's  eye  from  one  position  to  another, 
which  allows  him  to  watch  the  different  appearances  of 
light  and  shade  as  they  pass  gradually  into  each  other. 

Perhaps  the  greatest  advantage  of  one  over  the  other 
is  that  of  the  plane  mirror  over  the  concave  in  thus  making 
possible  the  more  complete  study  of  aberration  and  irregu- 
lar astigmatism. 


INDEX. 


ABERRATION,  17,  41,  56,  59,  84, 
99,  108. 

Accommodation, 86,  99,  106 

Accuracy 36,  40,  45,49 

Advantages,  5,   107 

Apparatus, ^S,  37)  71,  100 

Apparent  movement,. ..20,22  26,  31, 
50,  55, 

Applications, 71,  89 

Area  of  light.  ..23,  25,  26,  32,  47,  53 

Artificial  eye,   18 

Astigmatism,...  7,  17,  38,  41,  46,  56, 

76,  93,  107. 
Axonometer, 105 

BAND  appearance,... 7,  47,  78,  94 
Bowman 7,  10,  47 

Brilliancy  of  light, 33,  36 

Burnett, 104 

CHARNLEY, ; 9 
Chibret, 9,  n 

Concave  mirror,  9,  24,  39,  40,  44,  51, 
54,_  62.  64,  89,   109. 

Conditions  of  accuracy, 36 

Conical  cornea, 7,  65 

Contents, 3 

Couper 7,  104 

Cuignet 8,  9,  10 


FACIAL  light  area 23,  ''25 
Fantoscopie, 11 

Focal  lengths, 106 

Forbes, 9 

Form  of  light  area, 32,  47,  53 

Fundus-reflex  test, 11 

r^ALEZOWvSKI, 8,':ii 

HAINES, 104 
Hartridge, .11 

History, 7 

Hypermetropia 7,  24,  26,  72,  90 

TLLUSTRATlONS,..2o,  23,  25,  31, 
^     32,  48,  49.  55.  57,  61,  62,   67,  69, 

85,  102,  104,  105. 

Immediate  source, 23,  38 

Inclinometer 105 

Indicators, 105 

Inverted,  , 20,  61 

Irregular  astigmatism,  7,  41, "'56,  84, 

99- 

JACKSON, 9,  10,  81,  106 

'J     Juler, .'9 

KERATOSCOPIE,., 10 
Koroscopie, 11 


pvARK  ROOM, 36       I  AMBERT, 105 

L/     Derby, 37       L     Landolt, 11 

Learning  the  test, 13 

Lenses, 103,  104 

Light  area,..22,  25,  26,  28,  32,  ^8,  53 
Light  source, 23,  36,  71,  89,  102 


•37 
Difficulties 11 

Dioptric  Scale 1 06 

Dioptroscopie, 11 

Direction, 47,  55,  78,  97 

Distance, 42,43,  49,  89,  105,  108 

Donders, .-.7,  8       IWJAGNIFICATION  of  retina,. ..30 

Doyne, 104       J'l     Mengin 8 

Meridians, 46,  81,  97,  105 


CGGER II 

*-'     Emmetropia, 24,  26,  76,  93- 

Enlargement, 30 

Erect, 20,  61 


Mirror 37,  100 

Morton, 9 

Movements  of  light,    23,  25,  26,  28, 
31,  53,  61. 
(Ill) 


112  INDEX. 

Mydriatics io6       Retinal  area, 23,  25,  28,  32,  38 

Myopia, 7,  19,  24,  26,  74,  91       Retinal  enlargement, 30 

Retinophotoscopie, 11 

VjAME..... 10       Retinoscopy, 10 

1>      Negative  aberration,...  42,  59,       Retinoskiascopie 11 

^3?  108.  Reversal, 19,  34,  46 

r^BLIQUITY  of  lens, 70  QCISSORS  movement, 69 

^^      Oliver, II  O     Shade, 37,  102 

Optical  principles,... 19  Shadows, 26,  32,  47,  56,  60,  69 

Original  source  of  light 23,  38  Shadow-test, 11 

PARENT, 9,  10,  II       |!s^t-,^°l^' 37,  100 


•I        Plane  mirror,  ..9,  23,  38,  40,  44, 

50,  53,  60,  63,  71,  109 
Point  of  Reversal, 21,  34,  46 


Size  of  mirror, loi 

Smith ,  Priestl  ey , i  r 

Skiascopy, ii 


Position, ';4o/43',  49,'  7i!  89       ff""^^  ^^  ^^SK".  23,  36,  38,  71.  89 


Positive  aberration,. .41,  59,60,  108 
Practial  application, i^,  71 


Story, 9 

Study  of  test, 13 


Preface  ' J'  '   '    ^       Symmetrical  aberration,  41,  42,  58, 

prince,::"!;""":"!:":;:.:::;::::::::io5      59. 60, 63,  los. 

Principal  meridians, 46,  81,  97       -THORINGTON, 102,  103,  105 

Pupillary  shadows,...  26,  32,  47,  56,  1 

60,  69. 
Pupilloscopie 11       T  |  MBRASCOPY, 11 

RANDALL, 43 

Rate  of  movement,... 28,  31,66  W^^^^'^^  ZONE, 3.  59 

Real  movement, 22,  28 

Regular  astigmatism,... 7,  17,  38,  41,  TT /EILAND, 81 

46,76,93,107.  VV      Wiirdemann, 104 


* 


UNIVERSITY  OF  CALIFORNIA  LIBRARY 

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